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Quest Sql Optimizer For Oracle Keygenguru

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They are all assemblies installed into GAC and necessary for the product Quest SQL Optimizer for Oracle v8. Quest SQL Optimizer for Oracle 8.5.0.2031 + Crack Keygen/Serial Date added: Jan 2018 ScreenShot Review this Software Name. Email. Website Comment You may use these HTML tags and attributes:. ODOWNLOADX News. June 18 We have fixed. Submitting forms on the support site are temporary unavailable for schedule maintenance. If you need immediate assistance please contact technical support.We apologize for the inconvenience.

Note:The optimizer might not make the same decisions from one version of Oracle Database to the next. In recent versions, the optimizer might make different decisions, because better information is available.The output from the optimizer is an execution plan that describes an optimum method of execution. The plans shows the combination of the steps Oracle Database uses to execute a SQL statement. Each step either retrieves rows of data physically from the database or prepares them in some way for the user issuing the statement.For any SQL statement processed by Oracle, the optimizer performs the operations listed in. Table 11-1 Optimizer Operations OperationDescriptionEvaluation of expressions and conditionsThe optimizer first evaluates expressions and conditions containing constants as fully as possible.Statement transformationFor complex statements involving, for example, correlated subqueries or views, the optimizer might transform the original statement into an equivalent join statement.Choice of optimizer goalsThe optimizer determines the goal of optimization. See.Choice of access pathsFor each table accessed by the statement, the optimizer chooses one or more of the available access paths to obtain table data. See.Choice of join ordersFor a join statement that joins more than two tables, the optimizer chooses which pair of tables is joined first, and then which table is joined to the result, and so on.

See.The Oracle database provides query optimization. You can influence the optimizer's choices by setting the optimizer goal, and by gathering representative statistics for the query optimizer. The optimizer goal is either throughput or response time. See and.Sometimes, the application designer, who has more information about a particular application's data than is available to the optimizer, can choose a more effective way to execute a SQL statement. The application designer can use hints in SQL statements to instruct the optimizer about how a statement should be executed. 11.2 Choosing an Optimizer GoalBy default, the goal of the query optimizer is the best throughput. This means that it chooses the least amount of resources necessary to process all rows accessed by the statement.

Oracle can also optimize a statement with the goal of best response time. This means that it uses the least amount of resources necessary to process the first row accessed by a SQL statement.Choose a goal for the optimizer based on the needs of your application:.For applications performed in batch, such as Oracle Reports applications, optimize for best throughput. Usually, throughput is more important in batch applications, because the user initiating the application is only concerned with the time necessary for the application to complete. Response time is less important, because the user does not examine the results of individual statements while the application is running.For interactive applications, such as Oracle Forms applications or SQL.Plus queries, optimize for best response time. Usually, response time is important in interactive applications, because the interactive user is waiting to see the first row or first few rows accessed by the statement.The optimizer's behavior when choosing an optimization approach and goal for a SQL statement is affected by the following factors:.

Table 11-2 OPTIMIZERMODE Initialization Parameter Values ValueDescriptionALLROWSThe optimizer uses a cost-based approach for all SQL statements in the session regardless of the presence of statistics and optimizes with a goal of best throughput (minimum resource use to complete the entire statement). This is the default value.FIRSTROWS nThe optimizer uses a cost-based approach, regardless of the presence of statistics, and optimizes with a goal of best response time to return the first n number of rows; n can equal 1, 10, 100, or 1000.FIRSTROWSThe optimizer uses a mix of cost and heuristics to find a best plan for fast delivery of the first few rows.Note: Using heuristics sometimes leads the query optimizer to generate a plan with a cost that is significantly larger than the cost of a plan without applying the heuristic. FIRSTROWS is available for backward compatibility and plan stability; use FIRSTROWS n instead.You can change the goal of the query optimizer for all SQL statements in a session by changing the parameter value in initialization file or by the ALTER SESSION SET OPTIMIZERMODE statement. 11.2.3 Query Optimizer Statistics in the Data DictionaryThe statistics used by the query optimizer are stored in the data dictionary. You can collect exact or estimated statistics about physical storage characteristics and data distribution in these schema objects by using the DBMSSTATS package.To maintain the effectiveness of the query optimizer, you must have statistics that are representative of the data.

For table columns that contain values with large variations in number of duplicates, called skewed data, you should collect histograms.The resulting statistics provide the query optimizer with information about data uniqueness and distribution. Using this information, the query optimizer is able to compute plan costs with a high degree of accuracy. This enables the query optimizer to choose the best execution plan based on the least cost.If no statistics are available when using query optimization, the optimizer will do dynamic sampling depending on the setting of the OPTMIZERDYNAMICSAMPLING initialization parameter. This may cause slower parse times so for best performance, the optimizer should have representative optimizer statistics.

11.3.1 Enabling Query Optimizer FeaturesYou enable optimizer features by setting the OPTIMIZERFEATURESENABLE initialization parameter.OPTIMIZERFEATURESENABLE ParameterThe OPTIMIZERFEATURESENABLE parameter acts as an umbrella parameter for the query optimizer. This parameter can be used to enable a series of optimizer-related features, depending on the release.

It accepts one of a list of valid string values corresponding to the release numbers, such as 8.0.4, 8.1.7, and 9.2.0. For example, the following setting enables the use of the optimizer features in generating query plans in Oracle 10 g, Release 1.OPTIMIZERFEATURESENABLE=10.0.0;The OPTIMIZERFEATURESENABLE parameter was introduced with the main goal to allow customers to upgrade the Oracle server, yet preserve the old behavior of the query optimizer after the upgrade. For example, when you upgrade the Oracle server from release 8.1.5 to release 8.1.6, the default value of the OPTIMIZERFEATURESENABLE parameter changes from 8.1.5 to 8.1.6. This upgrade results in the query optimizer enabling optimization features based on 8.1.6, as opposed to 8.1.5.For plan stability or backward compatibility reasons, you might not want the query plans to change because of new optimizer features in a new release. In such a case, you can set the OPTIMIZERFEATURESENABLE parameter to an earlier version.

For example, to preserve the behavior of the query optimizer to release 8.1.5, set the parameter as follows:OPTIMIZERFEATURESENABLE=8.1.5;This statement disables all new optimizer features that were added in releases following release 8.1.5. If you upgrade to a new release and you want to enable the features available with that release, then you do not need to explicitly set the OPTIMIZERFEATURESENABLE initialization parameter. 11.3.2 Controlling the Behavior of the Query OptimizerThis section lists some initialization parameters that can be used to control the behavior of the query optimizer. These parameters can be used to enable various optimizer features in order to improve the performance of SQL execution.CURSORSHARINGThis parameter converts literal values in SQL statements to bind variables.

Converting the values improves cursor sharing and can affect the execution plans of SQL statements. The optimizer generates the execution plan based on the presence of the bind variables and not the actual literal values.DBFILEMULTIBLOCKREADCOUNTThis parameter specifies the number of blocks that are read in a single I/O during a full table scan or index fast full scan.

The optimizer uses the value of DBFILEMULTIBLOCKREADCOUNT to cost full table scans and index fast full scans. Larger values result in a cheaper cost for full table scans and can result in the optimizer choosing a full table scan over an index scan.

If this parameter is not set explicitly (or is set is 0), the default value corresponds to the maximum I/O size that can be efficiently performed and is platform-dependent.OPTIMIZERINDEXCACHINGThis parameter controls the costing of an index probe in conjunction with a nested loop. The range of values 0 to 100 for OPTIMIZERINDEXCACHING indicates percentage of index blocks in the buffer cache, which modifies the optimizer's assumptions about index caching for nested loops and IN-list iterators. A value of 100 infers that 100% of the index blocks are likely to be found in the buffer cache and the optimizer adjusts the cost of an index probe or nested loop accordingly. Use caution when using this parameter because execution plans can change in favor of index caching.OPTIMIZERINDEXCOSTADJThis parameter can be used to adjust the cost of index probes. The range of values is 1 to 10000. The default value is 100, which means that indexes are evaluated as an access path based on the normal costing model. A value of 10 means that the cost of an index access path is one-tenth the normal cost of an index access path.OPTIMIZERMODEThis initialization parameter sets the mode of the optimizer at instance startup.

The possible values are ALLROWS, FIRSTROWS n, and FIRSTROWS. For descriptions of these parameter values, see.PGAAGGREGATETARGETThis parameter automatically controls the amount of memory allocated for sorts and hash joins. Larger amounts of memory allocated for sorts or hash joins reduce the optimizer cost of these operations.

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See.STARTRANSFORMATIONENABLEDThis parameter, if set to true, enables the query optimizer to cost a star transformation for star queries. The star transformation combines the bitmap indexes on the various fact table columns. See Also:for detailed information on hintsThe query optimizer performs the following steps:.The optimizer generates a set of potential plans for the SQL statement based on available access paths and hints.The optimizer estimates the cost of each plan based on statistics in the data dictionary for the data distribution and storage characteristics of the tables, indexes, and partitions accessed by the statement.The cost is an estimated value proportional to the expected resource use needed to execute the statement with a particular plan.

The optimizer calculates the cost of access paths and join orders based on the estimated computer resources, which includes I/O, CPU, and memory.Serial plans with higher costs take more time to execute than those with smaller costs. When using a parallel plan, however, resource use is not directly related to elapsed time.The optimizer compares the costs of the plans and chooses the one with the lowest cost. 11.4.1.1 Transforming QueriesThe input to the query transformer is a parsed query, which is represented by a set of query blocks.

The query blocks are nested or interrelated to each other. The form of the query determines how the query blocks are interrelated to each other. The main objective of the query transformer is to determine if it is advantageous to change the form of the query so that it enables generation of a better query plan. Several different query transformation techniques are employed by the query transformer, including:.Any combination of these transformations can be applied to a given query. 11.4.1.1.1 View MergingEach view referenced in a query is expanded by the parser into a separate query block. The query block essentially represents the view definition, and therefore the result of a view. One option for the optimizer is to analyze the view query block separately and generate a view subplan.

The optimizer then processes the rest of the query by using the view subplan in the generation of an overall query plan. This technique usually leads to a suboptimal query plan, because the view is optimized separately from rest of the query.The query transformer then removes the potentially suboptimal plan by merging the view query block into the query block that contains the view. Most types of views are merged. When a view is merged, the query block representing the view is merged into the containing query block. Generating a subplan is no longer necessary, because the view query block is eliminated.Grant the MERGE ANY VIEW privilege to a user to enable the optimizer to use view merging for any query issued by the user. Grant the MERGE VIEW privilege to a user on specific views to enable the optimizer to use view merging for queries on these views.

These privileges are required only under certain conditions, such as when a view is not merged because the security checks fail. 11.4.1.1.4 Query Rewrite with Materialized ViewsA materialized view is like a query with a result that is materialized and stored in a table. When a user query is found compatible with the query associated with a materialized view, the user query can be rewritten in terms of the materialized view. This technique improves the execution of the user query, because most of the query result has been precomputed. The query transformer looks for any materialized views that are compatible with the user query and selects one or more materialized views to rewrite the user query. The use of materialized views to rewrite a query is cost-based. That is, the query is not rewritten if the plan generated without the materialized views has a lower cost than the plan generated with the materialized views.

11.4.1.2 Peeking of User-Defined Bind VariablesThe query optimizer peeks at the values of user-defined bind variables on the first invocation of a cursor. This feature enables the optimizer to determine the selectivity of any WHERE clause condition as if literals have been used instead of bind variables.To ensure the optimal choice of cursor for a given bind value, Oracle Database uses bind-aware cursor matching. The system monitors the data access performed by the query over time, depending on the bind values.

If bind peeking takes place, and a histogram is used to compute selectivity of the predicate containing the bind variable, then the cursor is marked as a bind-sensitive cursor. Whenever a cursor is determined to produce significantly different data access patterns depending on the bind values, that cursor is marked as bind-aware, and Oracle Database will switch to bind-aware cursor matching to select the cursor for that statement.

When bind-aware cursor matching is enabled, plans are selected based on the bind value and the optimizer's estimate of its selectivity. With bind-aware cursor matching, it is possible that a SQL statement with user-defined bind variable will have multiple execution plans, depending on the bind values.When bind variables are used in a SQL statement, it is assumed that cursor sharing is intended and that different invocations will use the same execution plan. If different invocations of the cursor will significantly benefit from different execution plans, then bind-aware cursor matching is required.

Bind peeking does not work for all clients, but a specific set of clients.Consider the following example:SELECT avg(e.salary), d.departmentnameFROM employees e, departments dWHERE e.jobid =:jobAND e.departmentid = d.departmentidGROUP BY d.departmentname;In this example, the column jobid is skewed because there are a lot more Sales Representatives ( jobid = 'SAREP') than there are Vice Presidents ( jobid = 'ADVP'). Therefore, the best plan for this query depends on the value of the bind variable. In this case, it is more efficient to use an index when the jobid is ADVP, and a full table scan when the jobid is SAREP.

The optimizer will peek at the first value ( 'ADVP') and choose an index, and the cursor will be marked as a bind-sensitive cursor. If the next time the query is executed and the bind value is MKREP (Marketing Representative) and this bind value has low selectivity, the optimizer may decide to mark the cursor as bind-aware and hard parse the statement to generate a new plan that performs a full table scan.The selectivity ranges, cursor information (such as whether a cursor is bind-aware or bind-sensitive), and execution statistics are available using the V$ views for extended cursor sharing. The V$SQLCSSTATISTICS view contains execution statistics for each cursor, and can be used for performance tuning by comparing the cursor executions generated with different bind sets. 11.4.1.3.1 SelectivityThe first measure, selectivity, represents a fraction of rows from a row set.

The row set can be a base table, a view, or the result of a join or a GROUP BY operator. The selectivity is tied to a query predicate, such as lastname = ' Smith', or a combination of predicates, such as lastname = ' Smith' AND jobtype = ' Clerk'. A predicate acts as a filter that filters a certain number of rows from a row set. Therefore, the selectivity of a predicate indicates how many rows from a row set will pass the predicate test. Selectivity lies in a value range from 0.0 to 1.0. A selectivity of 0.0 means that no rows will be selected from a row set, and a selectivity of 1.0 means that all rows will be selected.If no statistics are available then the optimizer either uses dynamic sampling or an internal default value, depending on the value of the OPTIMIZERDYNAMICSAMPLING initialization parameter.

Different internal defaults are used, depending on the predicate type. For example, the internal default for an equality predicate ( lastname = ' Smith') is lower than the internal default for a range predicate ( lastname ' Smith'). The estimator makes this assumption because an equality predicate is expected to return a smaller fraction of rows than a range predicate. See.When statistics are available, the estimator uses them to estimate selectivity. For example, for an equality predicate ( lastname = ' Smith'), selectivity is set to the reciprocal of the number n of distinct values of lastname, because the query selects rows that all contain one out of n distinct values. If a histogram is available on the lastname column, then the estimator uses it instead of the number of distinct values.

The histogram captures the distribution of different values in a column, so it yields better selectivity estimates. Having histograms on columns that contain skewed data (in other words, values with large variations in number of duplicates) greatly helps the query optimizer generate good selectivity estimates. 11.4.1.3.3 CostThe cost represents units of work or resource used. The query optimizer uses disk I/O, CPU usage, and memory usage as units of work. So, the cost used by the query optimizer represents an estimate of the number of disk I/Os and the amount of CPU and memory used in performing an operation. The operation can be scanning a table, accessing rows from a table by using an index, joining two tables together, or sorting a row set. The cost of a query plan is the number of work units that are expected to be incurred when the query is executed and its result produced.The access path determines the number of units of work required to get data from a base table.

The access path can be a table scan, a fast full index scan, or an index scan. During table scan or fast full index scan, multiple blocks are read from the disk in a single I/O operation. Therefore, the cost of a table scan or a fast full index scan depends on the number of blocks to be scanned and the multiblock read count value. The cost of an index scan depends on the levels in the B-tree, the number of index leaf blocks to be scanned, and the number of rows to be fetched using the rowid in the index keys. The cost of fetching rows using rowids depends on the index clustering factor. See.The join cost represents the combination of the individual access costs of the two row sets being joined, plus the cost of the join operation.

11.4.1.4 Generating PlansThe main function of the plan generator is to try out different possible plans for a given query and pick the one that has the lowest cost. Many different plans are possible because of the various combinations of different access paths, join methods, and join orders that can be used to access and process data in different ways and produce the same result.A join order is the order in which different join items, such as tables, are accessed and joined together. For example, in a join order of table1, table2, and table3, table table1 is accessed first. Next, table2 is accessed, and its data is joined to table1 data to produce a join of table1 and table2. Finally, table3 is accessed, and its data is joined to the result of the join between table1 and table2.The plan for a query is established by first generating subplans for each of the nested subqueries and unmerged views.

Each nested subquery or unmerged view is represented by a separate query block. The query blocks are optimized separately in a bottom-up order. That is, the innermost query block is optimized first, and a subplan is generated for it.

The outermost query block, which represents the entire query, is optimized last.The plan generator explores various plans for a query block by trying out different access paths, join methods, and join orders. The number of possible plans for a query block is proportional to the number of join items in the FROM clause. This number rises exponentially with the number of join items.The plan generator uses an internal cutoff to reduce the number of plans it tries when finding the one with the lowest cost. The cutoff is based on the cost of the current best plan. If the current best cost is large, then the plan generator tries harder (in other words, explores more alternate plans) to find a better plan with lower cost. If the current best cost is small, then the plan generator ends the search swiftly, because further cost improvement will not be significant.The cutoff works well if the plan generator starts with an initial join order that produces a plan with cost close to optimal.

Finding a good initial join order is a difficult problem. 11.4.2 Reading and Understanding Execution PlansTo execute a SQL statement, Oracle might need to perform many steps.

Each of these steps either retrieves rows of data physically from the database or prepares them in some way for the user issuing the statement. The combination of the steps Oracle uses to execute a statement is called an execution plan. An execution plan includes an access path for each table that the statement accesses and an ordering of the tables (the join order) with the appropriate join method.

11.4.2.1 Overview of EXPLAIN PLANYou can examine the execution plan chosen by the optimizer for a SQL statement by using the EXPLAIN PLAN statement. When the statement is issued, the optimizer chooses an execution plan and then inserts data describing the plan into a database table. Simply issue the EXPLAIN PLAN statement and then query the output table.These are the basics of using the EXPLAIN PLAN statement:.Use the SQL script UTLXPLAN. SQL to create a sample output table called PLANTABLE in your schema.

See.Include the EXPLAIN PLAN FOR clause before the SQL statement. See.After issuing the EXPLAIN PLAN statement, use one of the scripts or package provided by Oracle Database to display the most recent plan table output. See.The execution order in EXPLAIN PLAN output begins with the line that is the furthest indented to the right.

The next step is the parent of that line. If two lines are indented equally, then the top line is normally executed first. See Also:Each step of the execution plan returns a set of rows that either is used by the next step or, in the last step, is returned to the user or application issuing the SQL statement.

A set of rows returned by a step is called a row set.The numbering of the step Ids reflects the order in which they are displayed in response to the EXPLAIN PLAN statement. 11.5 Understanding Access Paths for the Query OptimizerAccess paths are ways in which data is retrieved from the database.

In general, index access paths should be used for statements that retrieve a small subset of table rows, while full scans are more efficient when accessing a large portion of the table. Online transaction processing (OLTP) applications, which consist of short-running SQL statements with high selectivity, often are characterized by the use of index access paths. Decision support systems, on the other hand, tend to use partitioned tables and perform full scans of the relevant partitions.This section describes the data access paths that can be used to locate and retrieve any row in any table. 11.5.1 Full Table ScansThis type of scan reads all rows from a table and filters out those that do not meet the selection criteria. During a full table scan, all blocks in the table that are under the high water mark are scanned. The high water mark indicates the amount of used space, or space that had been formatted to receive data.

Each row is examined to determine whether it satisfies the statement's WHERE clause.When Oracle performs a full table scan, the blocks are read sequentially. Because the blocks are adjacent, I/O calls larger than a single block can be used to speed up the process. The size of the read calls range from one block to the number of blocks indicated by the initialization parameter DBFILEMULTIBLOCKREADCOUNT.

Using multiblock reads means a full table scan can be performed very efficiently. Each block is read only once.contains an example of a full table scan on the employees table.

11.5.1.3 Full Table Scan HintsUse the hint FULL( table alias ) to instruct the optimizer to use a full table scan. For more information on the FULL hint, see.You can use the CACHE and NOCACHE hints to indicate where the retrieved blocks are placed in the buffer cache.

The CACHE hint instructs the optimizer to place the retrieved blocks at the most recently used end of the LRU list in the buffer cache when a full table scan is performed.Small tables are automatically cached according to the criteria in. Table 11-4 Table Caching Criteria Table SizeSize CriteriaCachingSmallNumber of blocks 10% of total cached blocksNot cachedAutomatic caching of small tables is disabled for tables that are created or altered with the CACHE attribute. 11.5.2 Rowid ScansThe rowid of a row specifies the datafile and data block containing the row and the location of the row in that block. Locating a row by specifying its rowid is the fastest way to retrieve a single row, because the exact location of the row in the database is specified.To access a table by rowid, Oracle first obtains the rowids of the selected rows, either from the statement's WHERE clause or through an index scan of one or more of the table's indexes. Oracle then locates each selected row in the table based on its rowid.In, an index scan is performed the jobs and departments tables.

The rowids retrieved are used to return the row data. 11.5.3 Index ScansIn this method, a row is retrieved by traversing the index, using the indexed column values specified by the statement. An index scan retrieves data from an index based on the value of one or more columns in the index.

To perform an index scan, Oracle searches the index for the indexed column values accessed by the statement. If the statement accesses only columns of the index, then Oracle reads the indexed column values directly from the index, rather than from the table.The index contains not only the indexed value, but also the rowids of rows in the table having that value. Therefore, if the statement accesses other columns in addition to the indexed columns, then Oracle can find the rows in the table by using either a table access by rowid or a cluster scan.An index scan can be one of the following types:. 11.5.3.1 Assessing I/O for Blocks, not RowsOracle does I/O by blocks. Therefore, the optimizer's decision to use full table scans is influenced by the percentage of blocks accessed, not rows. This is called the index clustering factor. If blocks contain single rows, then rows accessed and blocks accessed are the same.However, most tables have multiple rows in each block.

Consequently, the desired number of rows could be clustered together in a few blocks, or they could be spread out over a larger number of blocks.Although the clustering factor is a property of the index, the clustering factor actually relates to the spread of similar indexed column values within data blocks in the table. A lower clustering factor indicates that the individual rows are concentrated within fewer blocks in the table. Conversely, a high clustering factor indicates that the individual rows are scattered more randomly across blocks in the table. Therefore, a high clustering factor means that it costs more to use a range scan to fetch rows by rowid, because more blocks in the table need to be visited to return the data. Shows how the clustering factor can affect cost.

Example 11-3 Effects of Clustering Factor on CostAssume the following situation:.There is a table with 9 rows.There is a non-unique index on col1 for table.The c1 column currently stores the values A, B, and C.The table only has three Oracle blocks.Case 1: The index clustering factor is low for the rows as they are arranged in the following diagram.Block 1 Block 2 Block 3-A A A B B B C C CThis is because the rows that have the same indexed column values for c1 are located within the same physical blocks in the table. The cost of using a range scan to return all of the rows that have the value A is low, because only one block in the table needs to be read.Case 2: If the same rows in the table are rearranged so that the index values are scattered across the table blocks (rather than collocated), then the index clustering factor is higher.Block 1 Block 2 Block 3-A B C A B C A B CThis is because all three blocks in the table must be read in order to retrieve all rows with the value A in col1. 11.5.3.3 Index Range ScansAn index range scan is a common operation for accessing selective data.

It can be bounded (bounded on both sides) or unbounded (on one or both sides). Data is returned in the ascending order of index columns.

Multiple rows with identical values are sorted in ascending order by rowid.If data must be sorted by order, then use the ORDER BY clause, and do not rely on an index. If an index can be used to satisfy an ORDER BY clause, then the optimizer uses this option and avoids a sort.In, the order has been imported from a legacy system, and you are querying the order by the reference used in the legacy system. Assume this reference is the orderdate. 11.5.3.3.1 When the Optimizer Uses Index Range ScansThe optimizer uses a range scan when it finds one or more leading columns of an index specified in conditions, such as the following:.col1 =:b1.col1:b1.AND combination of the preceding conditions for leading columns in the index.col1 like 'ASD%' wild-card searches should not be in a leading position otherwise the condition col1 like '%ASD' does not result in a range scan.Range scans can use unique or non-unique indexes. Range scans avoid sorting when index columns constitute the ORDER BY/ GROUP BY clause.

Quest Sql Optimizer For Oracle Keygenguru Version

11.5.3.5 Index Skip ScansIndex skip scans improve index scans by nonprefix columns. Often, scanning index blocks is faster than scanning table data blocks.Skip scanning lets a composite index be split logically into smaller subindexes. In skip scanning, the initial column of the composite index is not specified in the query. In other words, it is skipped.The number of logical subindexes is determined by the number of distinct values in the initial column.

Skip scanning is advantageous if there are few distinct values in the leading column of the composite index and many distinct values in the nonleading key of the index. Example 11-5 Index Skip ScanConsider, for example, a table employees ( sex, employeeid, address) with a composite index on ( sex, employeeid). Splitting this composite index would result in two logical subindexes, one for M and one for F.For this example, suppose you have the following index data:('F',98)('F',100)('F',102)('F',104)('M',101)('M',103)('M',105)The index is split logically into the following two subindexes:.The first subindex has the keys with the value F.The second subindex has the keys with the value M. 11.5.3.6 Full ScansA full index scan eliminates a sort operation, because the data is ordered by the index key. It reads the blocks singly.

A full scan is used in any of the following situations:.An ORDER BY clause that meets the following requirements is present in the query:.All of the columns in the ORDER BY clause must be in the index.The order of the columns in the ORDER BY clause must match the order of the leading index columns.The ORDER BY clause can contain all of the columns in the index or a subset of the columns in the index.The query requires a sort merge join. A full index scan can be done instead of doing a full table scan followed by a sort if the query meets the following requirements:.All of the columns referenced in the query must be in the index.The order of the columns referenced in the query must match the order of the leading index columns.The query can contain all of the columns in the index or a subset of the columns in the index.A GROUP BY clause is present in the query, and the columns in the GROUP BY clause are present in the index. The columns do not need to be in the same order in the index and the GROUP BY clause.

The GROUP BY clause can contain all of the columns in the index or a subset of the columns in the index. 11.5.3.7 Fast Full Index ScansFast full index scans are an alternative to a full table scan when the index contains all the columns that are needed for the query, and at least one column in the index key has the NOT NULL constraint. A fast full scan accesses the data in the index itself, without accessing the table. It cannot be used to eliminate a sort operation, because the data is not ordered by the index key. It reads the entire index using multiblock reads, unlike a full index scan, and can be parallelized.You can specify fast full index scans with the initialization parameter OPTIMIZERFEATURESENABLE or the INDEXFFS hint.

Fast full index scans cannot be performed against bitmap indexes.A fast full scan is faster than a normal full index scan in that it can use multiblock I/O and can be parallelized just like a table scan. 11.5.6 Sample Table ScansA sample table scan retrieves a random sample of data from a simple table or a complex SELECT statement, such as a statement involving joins and views. This access path is used when a statement's FROM clause includes the SAMPLE clause or the SAMPLE BLOCK clause. To perform a sample table scan when sampling by rows with the SAMPLE clause, Oracle reads a specified percentage of rows in the table. To perform a sample table scan when sampling by blocks with the SAMPLE BLOCK clause, Oracle reads a specified percentage of table blocks.uses a sample table scan to access 1% of the employees table, sampling by blocks. Example 11-6 Sample Table ScanSELECT.FROM employees SAMPLE BLOCK (1);The EXPLAIN PLAN output for this statement might look like this:- Id Operation Name Rows Bytes Cost (%CPU) - 0 SELECT STATEMENT 1 68 3 (34) 1 TABLE ACCESS SAMPLE EMPLOYEES 1 68 3 (34).

11.5.7 How the Query Optimizer Chooses an Access PathThe query optimizer chooses an access path based on the following factors:.The available access paths for the statement.The estimated cost of executing the statement, using each access path or combination of pathsTo choose an access path, the optimizer first determines which access paths are available by examining the conditions in the statement's WHERE clause and its FROM clause. The optimizer then generates a set of possible execution plans using available access paths and estimates the cost of each plan, using the statistics for the index, columns, and tables accessible to the statement. Finally, the optimizer chooses the execution plan with the lowest estimated cost.When choosing an access path, the query optimizer is influenced by the following:.Optimizer HintsYou can instruct the optimizer to use a specific access path using a hint, except when the statement's FROM clause contains SAMPLE or SAMPLE BLOCK. 11.6.1 How the Query Optimizer Executes Join StatementsTo choose an execution plan for a join statement, the optimizer must make these interrelated decisions:.Access PathsAs for simple statements, the optimizer must choose an access path to retrieve data from each table in the join statement.Join MethodTo join each pair of row sources, Oracle must perform a join operation.

Join methods include nested loop, sort merge, cartesian, and hash joins.Join OrderTo execute a statement that joins more than two tables, Oracle joins two of the tables and then joins the resulting row source to the next table. This process is continued until all tables are joined into the result. 11.6.2 How the Query Optimizer Chooses Execution Plans for JoinsThe query optimizer considers the following when choosing an execution plan:.The optimizer first determines whether joining two or more tables definitely results in a row source containing at most one row. The optimizer recognizes such situations based on UNIQUE and PRIMARY KEY constraints on the tables. If such a situation exists, then the optimizer places these tables first in the join order.

The optimizer then optimizes the join of the remaining set of tables.For join statements with outer join conditions, the table with the outer join operator must come after the other table in the condition in the join order. The optimizer does not consider join orders that violate this rule.

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Similarly, when a subquery has been converted into an antijoin or semijoin, the tables from the subquery must come after those tables in the outer query block to which they were connected or correlated. However, hash antijoins and semijoins are able to override this ordering condition in certain circumstances.With the query optimizer, the optimizer generates a set of execution plans, according to possible join orders, join methods, and available access paths. The optimizer then estimates the cost of each plan and chooses the one with the lowest cost.

The optimizer estimates costs in the following ways:.The cost of a nested loops operation is based on the cost of reading each selected row of the outer table and each of its matching rows of the inner table into memory. The optimizer estimates these costs using the statistics in the data dictionary.The cost of a sort merge join is based largely on the cost of reading all the sources into memory and sorting them.The cost of a hash join is based largely on the cost of building a hash table on one of the input sides to the join and using the rows from the other of the join to probe it.The optimizer also considers other factors when determining the cost of each operation. For example:.A smaller sort area size is likely to increase the cost for a sort merge join because sorting takes more CPU time and I/O in a smaller sort area. See for information on sizing of SQL work areas.A larger multiblock read count is likely to decrease the cost for a sort merge join in relation to a nested loop join. If a large number of sequential blocks can be read from disk in a single I/O, then an index on the inner table for the nested loop join is less likely to improve performance over a full table scan.

The multiblock read count is specified by the initialization parameter DBFILEMULTIBLOCKREADCOUNT.With the query optimizer, the optimizer's choice of join orders can be overridden with the ORDERED hint. If the ORDERED hint specifies a join order that violates the rule for an outer join, then the optimizer ignores the hint and chooses the order.

Also, you can override the optimizer's choice of join method with hints. 11.6.3 Nested Loop JoinsNested loop joins are useful when small subsets of data are being joined and if the join condition is an efficient way of accessing the second table.It is very important to ensure that the inner table is driven from (dependent on) the outer table. If the inner table's access path is independent of the outer table, then the same rows are retrieved for every iteration of the outer loop, degrading performance considerably. In such cases, hash joins joining the two independent row sources perform better. 11.6.3.1 Original and New Implementation for Nested Loop JoinsOracle Database 11 g Release 1 (11.1) introduces a new implementation for nested loop joins. As a result, execution plans that include nested loops might appear different than they did in previous releases of Oracle Database. Both the new implementation and the original implementation for nested loop joins are possible in Oracle Database 11 g Release 1 (11.1).

So, when analyzing execution plans, it is important to understand that the number of NESTED LOOPS join row sources might be different. 11.6.3.1.2 New Implementation for Nested Loop JoinsOracle Database 11 g Release 1 (11.1) introduces a new implementation for nested loop joins to reduce overall latency for physical I/O. When an index or a table block is not in the buffer cache and is needed to process the join, a physical I/O is required. In Oracle Database 11 g Release 1 (11.1), Oracle Database can batch multiple physical I/O requests and process them using a vector I/O instead of processing them one at a time. As part of the new implementation for nested loop joins, two NESTED LOOPS join row sources might appear in the execution plan where only one would have appeared in prior releases.

For

In such cases, Oracle Database allocates one NESTED LOOPS join row source to join the values from the table on the outer side of the join with the index on the inner side. A second row source is allocated to join the result of the first join, which includes the rowids stored in the index, with the table on the inner side of the join.Consider the query in. 11.6.3.2 When the Optimizer Uses Nested Loop JoinsThe optimizer uses nested loop joins when joining small number of rows, with a good driving condition between the two tables. You drive from the outer loop to the inner loop, so the order of tables in the execution plan is important.The outer loop is the driving row source. It produces a set of rows for driving the join condition.

The row source can be a table accessed using an index scan or a full table scan. Also, the rows can be produced from any other operation. For example, the output from a nested loop join can be used as a row source for another nested loop join.The inner loop is iterated for every row returned from the outer loop, ideally by an index scan. If the access path for the inner loop is not dependent on the outer loop, then you can end up with a Cartesian product; for every iteration of the outer loop, the inner loop produces the same set of rows. Therefore, you should use other join methods when two independent row sources are joined together.

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11.6.3.3 Nested Loop Join HintsIf the optimizer is choosing to use some other join method, you can use the USENL( table1 table2) hint, where table1 and table2 are the aliases of the tables being joined.For some SQL examples, the data is small enough for the optimizer to prefer full table scans and use hash joins. This is the case for the SQL example shown in. However, you can add a USENL to instruct the optimizer to change the join method to nested loop. For more information on the USENL hint, see.

11.6.5 Sort Merge JoinsSort merge joins can be used to join rows from two independent sources. Hash joins generally perform better than sort merge joins. On the other hand, sort merge joins can perform better than hash joins if both of the following conditions exist:.The row sources are sorted already.A sort operation does not have to be done.However, if a sort merge join involves choosing a slower access method (an index scan as opposed to a full table scan), then the benefit of using a sort merge might be lost.Sort merge joins are useful when the join condition between two tables is an inequality condition (but not a nonequality) like, or =. Sort merge joins perform better than nested loop joins for large data sets. You cannot use hash joins unless there is an equality condition.In a merge join, there is no concept of a driving table.

The join consists of two steps:.Sort join operation: Both the inputs are sorted on the join key.Merge join operation: The sorted lists are merged together.If the input is already sorted by the join column, then a sort join operation is not performed for that row source. However, a sort merge join always creates a positionable sort buffer for the right side of the join so that it can seek back to the last match in the case where duplicate join key values come out of the left side of the join. 11.6.5.2 Sort Merge Join HintsTo instruct the optimizer to use a sort merge join, apply the USEMERGE hint. You might also need to give hints to force an access path.There are situations where it is better to override the optimizer with the USEMERGE hint. For example, the optimizer can choose a full scan on a table and avoid a sort operation in a query. However, there is an increased cost because a large table is accessed through an index and single block reads, as opposed to faster access through a full table scan.For more information on the USEMERGE hint, see. 11.6.7.1 Nested Loop Outer JoinsThis operation is used when an outer join is used between two tables.

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The outer join returns the outer (preserved) table rows, even when there are no corresponding rows in the inner (optional) table.In a regular outer join, the optimizer chooses the order of tables (driving and driven) based on the cost. However, in a nested loop outer join, the order of tables is determined by the join condition. The outer table, with rows that are being preserved, is used to drive to the inner table.The optimizer uses nested loop joins to process an outer join in the following circumstances:.It is possible to drive from the outer table to inner table.Data volume is low enough to make the nested loop method efficient.For an example of a nested loop outer join, you can add the USENL hint to to instruct the optimizer to use a nested loop. For example:SELECT /.+ USENL(c o)./ custlastname, sum(nvl2(o.customerid,0,1)) 'Count'. 11.6.7.2 Hash Join Outer JoinsThe optimizer uses hash joins for processing an outer join if the data volume is high enough to make the hash join method efficient or if it is not possible to drive from the outer table to inner table.The order of tables is determined by the cost. The outer table, including preserved rows, may be used to build the hash table, or it may be used to probe one.shows a typical hash join outer join query.

In this example, all the customers with credit limits greater than 1000 are queried. An outer join is needed so that you do not miss the customers who do not have any orders. 11.6.7.3 Sort Merge Outer JoinsWhen an outer join cannot drive from the outer (preserved) table to the inner (optional) table, it cannot use a hash join or nested loop joins. Then it uses the sort merge outer join for performing the join operation.The optimizer uses sort merge for an outer join:.If a nested loop join is inefficient. A nested loop join can be inefficient because of data volumes.The optimizer finds it is cheaper to use a sort merge over a hash join because of sorts already required by other operations.

11.6.7.4 Full Outer JoinsA full outer join acts like a combination of the left and right outer joins. In addition to the inner join, rows from both tables that have not been returned in the result of the inner join are preserved and extended with nulls. In other words, full outer joins let you join tables together, yet still show rows that do not have corresponding rows in the joined tables.The query in retrieves all departments and all employees in each department, but also includes:.Any employees without departments.Any departments without employees. Example 11-10 Full Outer JoinSELECT d.departmentid, e.employeeidFROM employees eFULL OUTER JOIN departments dON e.departmentid = d.departmentidORDER BY d.departmentid;The statement produces the following output:DEPARTMENTID EMPLOYEEID- -10 0 0 11530 116.07125 rows selected.Starting with Oracle Database 11 g Release 1 (11.1), Oracle Database automatically uses a native execution method based on a hash join for executing full outer joins whenever possible. When the new method is used to execute a full outer join, the execution plan for the query contains HASH JOIN FULL OUTER. Shows the execution plan for the query in.

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ComponentKnown IssueChange RequestInstallationIf you have installed Microsoft.NET Framework 2.0 Beta, you will receive the errormessage: 'The installation of component Microsoft Net Framework 20 has failed.' Get thelatest product information, find helpful resources, and join a discussionwith the Quest SQL Optimizer team andother community members. Join the SQL Optimizer community at. Contacting Quest Software: EmailMailQuest Software, Inc.World Headquarters5 Polaris WayAliso Viejo, CA 92656USAWebRefer to our Web site for regional and international office information. Contacting Quest Support:Quest Support is available to customers who have a trial version of a Quest product or who have purchased a commercial version and have a valid maintenance contract.Quest Support provides around the clock coverage with SupportLink, our web self-service. Visit SupportLink at.From SupportLink, you can do the following:. Quickly find thousands of solutions (Knowledgebase articles/documents).

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Log and update your case, and check its status.View the Global Support Guide for a detailed explanation of support programs, online services, contact information, and policy and procedures.The guide is available at:.This document contains proprietary information protected by copyright. The software described in this guide is furnished under a software license or nondisclosure agreement. This software may be used or copied only in accordance with the terms of the applicable agreement. No part of this guide may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording for any purpose other than the purchaser’s personal use without the written permission of Quest Software, Inc.© 2007 Quest Software, Inc.