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Specifying optimization parameters at the statement level

You can customize the values of certain optimization-related subsystem parameters and bind options for all instances of particular SQL statements within particular scopes by creating statement-level optimization parameters.

Before you begin

Begin program-specific programming interface information.
The following prerequisites are met:
  • Prepare to manage access paths.
  • You have one of the following authorities:
    • SQLADM
    • SYSOPR
    • SYSCTRL
    • SYSADM
  • Start of changeAn instance of the DSN_USERQUERY_TABLE user table is created under your schema, or under a separate schema for input tables. For more information about using tables under a separate schema see Creating input EXPLAIN tables under a separate schemaEnd of changeYou can find sample CREATE statements for the tables and associated indexes in members DSNTESC and DSNTESH of the prefix.SDSNSAMP library.
  • All object names are UPPER CASE in the query text of the SQL statements.
  • Start of changeThe package that contains the statement was created by a BIND PACKAGE statement. Statement-level methods for influencing access paths are not supported for statements in packages that are created by other statements, such as CREATE FUNCTION, CREATE TRIGGER, and CREATE PROCEDURE statements.End of change
  • An instance of PLAN_TABLE is created under your schema.
  • An index is created on the following PLAN_TABLE columns:
    • QUERYNO
    • APPLNAME
    • PROGNAME
    • VERSION
    • COLLID
    • OPTHINT

    A sample statement that creates the index is included in member DSNTESC of the SDSNSAMP library.

About this task

Statement-level optimization parameters use matching of the statement text to apply the specified optimization parameter value to all instances of a statement within one of the following scopes:

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  • System-wide
  • From any version of particular collection and package
  • From a particular version of a collection and package
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Procedure

To specify statement-level optimization parameters:

  1. INSERT rows into the DSN_USERQUERY_TABLE table.
    1. Insert values in the following columns to specify the SQL statement and context for the optimization parameter:
      QUERYNO
      Specify any value that does not correlate to PLAN_TABLE rows and does not already exist in another DSN_USERQUERY_TABLE row. The QUERYNO value is used only for the primary key of DSN_USERQUERY_TABLE.
      Start of changeSCHEMAEnd of change
      Start of changeIf the SQL statement contains unqualified object names that might resolve to different default schemas, insert the schema name that identifies the unqualified database objects. If the statement contains unqualified objects names because it might apply to different schemas at different times, you must create separate hints or overrides for each possible SCHEMA value. If the statement contains only fully qualified object names, the SCHEMA value is not required. However, you can still insert a SCHEMA value to help you identify that the hint relates to a certain schema.End of change
      QUERY_TEXT
      Insert the text of the statement whose access path you want to influence.

      The text that you provide must match the statement text that DB2 uses when binding static SQL statements and preparing dynamic SQL statements. For more information about how to enable successful text matching, see Populating query text for statement-level matching.

      HINT_SCOPE
      Insert a value to specify that context in which to match the statement.
      0
      System wide. DB2 uses only the text of the SQL statement and the value of the SCHEMA column, when it contains a value, to determine whether the statement matches.
      1
      Package-level. DB2 uses the values of the COLLECTION, PACKAGE, and VERSION columns to determine whether the statement matches.
      COLLECTION
      Insert the collection name of the package. This value is required only when the value of HINT_SCOPE is 1.

      Start of changeWhen the value of HINT_SCOPE is 0, the value is optional, and when a value is specified DB2 issues an error message when you bind the query if the matching value is not found in the SYSIBM.SYSPACKAGE catalog table. When HINT_SCOPE is 0, either specify both COLLECTION and PACKAGE or leave both fields blank.End of change

      Start of changeFor static SQL statements and dynamic SQL statements that use the DYNAMICRULES(BIND) option, you might need to specify the value of this column so that DB2 can retrieve the correct application default values from the SYSIBM.SYSPACKSTMT catalog table.End of change

      PACKAGE
      Insert the name of the package. This value is required only when the value of HINT_SCOPE is 1.

      Start of changeWhen the value of HINT_SCOPE is 0, the value is optional, and when a value is specified DB2 issues an error message when you bind the query if the matching value is not found in the SYSIBM.SYSPACKAGE catalog table. When HINT_SCOPE is 0, either specify both COLLECTION and PACKAGE or leave both fields blank.End of change

      Start of changeFor static SQL statements and dynamic SQL statements that use the DYNAMICRULES(BIND) option, you might need to specify the value of this column so that DB2 can retrieve the correct application default values from the SYSIBM.SYSPACKSTMT catalog table.End of change

      Start of changeThe package-specific scope is intended primarily to support the staging, validation, and testing of statement-level hints, before they are deployed with a system-wide scope.End of change

      VERSION
      Insert the version identifier of the package or '*'. A value in this column is required only when the value of HINT_SCOPE is 1. When you specify '*' for the VERSION column, DB2 does not require matching of the VERSION column for statement matching.

      When the value of HINT_SCOPE is 0, this value is optional. When a value is specified DB2 issues an error message when you bind the query if the matching value is not found in the SYSIBM.SYSPACKAGE catalog table.

      Start of changeFor static SQL statements and dynamic SQL statements that use the DYNAMICRULES(BIND) option, you might need to specify the value of this column so that DB2 can retrieve the correct application default values from the SYSIBM.SYSPACKSTMT catalog table.End of change

    2. Insert values that define the optimization parameters. Statement-level optimization parameters are created only when the QUERYNO value that you specify in DSN_USERQUERY_TABLE does not correlate to existing PLAN_TABLE rows. You can specify settings for the following optimization parameters and options:
      • REOPT bind option
      • STARJOIN subsystem parameter
      • PARAMDEG subsystem parameter (MAX_PAR_DEGREE column)
      • CDSSRDEF subsystem parameter (DEF_CURR_DEGREE column)
      • SJTABLES subsystem parameter
      Start of changeFor example, you might execute the following SQL statement to insert a row into DSN_USERQUERY_TABLE that creates an optimization parameter hint:
      INSERT INTO DSN_USERQUERY_TABLE 
( QUERYNO, SCHEMA, HINT_SCOPE, QUERY_TEXT,
  USERFILTER, OTHER_OPTIONS,
  COLLECTION, PACKAGE, VERSION, 
  REOPT, STARJOIN, MAX_PAR_DEGREE, 
  DEF_CURR_DEGREE, SJTABLES, OTHER_PARMS )   
  VALUES  
  (100, 'MYSCHEMA_1', 0,
   'DECLARE C06 CURSOR FOR
     SELECT N_NAME, COUNT(*)
       FROM ORDER, CUSTOMER, NATION_NP
       WHERE C_NATIONKEY = N_NATIONKEY
         AND C_CUSTKEY = O_CUSTKEY
         AND N_REGIONKEY = :H
         AND O_ORDERDATE BETWEEN ''1998-01-01'' AND ''1998-03-31''
       GROUP BY N_NAME',                               
   '', '',        
   '', '', '',            
   'Y', '', -1, 
   '', -1, '');

      The result is that DB2 uses the REOPT(ALWAYS) optimization parameter for instances of the specified statement.

      End program-specific programming interface information.
      End of change
  2. Issue a BIND QUERY command. Start of changeYou must omit the LOOKUP option or specify LOOKUP(NO).End of change DB2 takes the input from every DSN_USERQUERY_TABLE row, and from related input tables, and inserts data into the following catalog tables:
    • SYSIBM.SYSQUERY
    • SYSIBM.SYSQUERYOPTS

    The QUERYID column correlates rows in these tables.

Results

The catalog table rows for static SQL statements are validated and applied when you rebind the package that contains the statements. Catalog table rows for dynamic SQL statements are validated and enforced when the statements are prepared.

What to do next

Consider taking the following actions:

  1. Validate that the appropriate catalog table rows have been created:
    1. Insert row into the DSN_USERQUERY_TABLE table that contain values in the QUERY_TEXT and SCHEMA columns.
    2. Issue the following command:
      BIND QUERY LOOKUP(YES)
      DB2 issues the following messages to indicate whether the catalog tables contain valid rows that correspond to the DSN_USERQUERY_TABLE rows.Start of change
      • A DSNT280I message for each DSN_USERQUERY_TABLE row that has matching rows in the catalog tables.
      • A DSNT281I message for each DSN_USERQUERY_TABLE row that does not have matching rows in the catalog table.
      • A single DSNT290I message if some matching rows were found in the catalog tables or a DSNT291I message if no matching rows were found.
      End of change DB2 also updates the value of QUERYID column in the DSN_USERQUERY_TABLE table to match the value from the matching rows in the SYSIBM.SYSQUERY catalog table.
  2. Delete the DSN_USERQUERY_TABLE rows to prevent the replacement of existing catalog table rows when you issue subsequent BIND QUERY commands. When you issue a BIND_QUERY command, catalog tables rows are created or replaced for every row in DSN_USERQUERY_TABLE row. Changes to data in other input tables might have unintended consequences if old rows remain in the DSN_USERQUERY_TABLE and you issue the BIND_QUERY command again.
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