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

The following prerequisites are met:

• Prepare to manage access paths.
• You have one of the following authorities:
  • SQLADM
  • SYSOPR
  • SYSCTRL
  • SYSADM
• An 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 schemaYou can find sample CREATE statements for the tables and associated indexes in members DSNTESC and DSNTESH of the prefix.SDSNSAMP library.
• The 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.
• 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.

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.

      SCHEMA

      If 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.

      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 ID of the package. This value is required only when the value of HINT_SCOPE is 1.

      When 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.

      For 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.

      PACKAGE

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

      When 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.

      For 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.

      The 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.

      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.

      For 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.

      SELECTVTY_OVERRIDE

      Specify a value to indicate whether selectivity overrides are specified. Unless you specifically want to enable both option overrides and selectivity overrides for the same statement, specify 'N' in this column. If you want to specify both types of overrides, specify 'Y'.

      ACCESSPATH_HINT

      Specify a value of 'N' to indicate that no access path is specified. You cannot specify both access paths and selectivity overrides for the same statements.

      OPTION_OVERRIDE

      Specify a value of 'Y' to indicate that statement-level optimization parameters are specified.

      Important: A previous format of DSN_USERQUERY_TABLE that does not contain the SELECTVTY_OVERRIDE, ACCESSPATH_HINT, and OPTION_OVERRIDE columns is also supported for compatibility. However, the recommendation is to use the most recent format of DSN_USERQUERY_TABLE and specify values in each of these columns. By specifying these values, you simplify the process of using only intended methods to influence access paths.
   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
      For 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,
      SELECTVTY_OVERRIDE, ACCESSPATH_HINT, OPTION_OVERRIDE
      )   
        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',                               
         'USER001', '',        
         '', '', '',            
         'Y', '', -1, 
         '', -1, '',
      'N', 'N', 'Y'
      );     

      The result is that Db2 uses the REOPT(ALWAYS) optimization parameter for instances of the specified statement.

      
2. Issue a BIND QUERY command.
   You must omit the LOOKUP option or specify LOOKUP(NO).
   If you created input tables under a dedicated schema, specify the EXPLAININPUTSCHEMA bind option and the name of the schema.
   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.