Specifying access paths at the statement level

You can suggest that DB2® uses a certain access path for all instances of a particular SQL statement within a specified scope by creating statement-level access paths.

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
Instances of the following user tables exist under your schema, or under a separate schema for input tables:
- DSN_USERQUERY_TABLE
- PLAN_TABLE
For more information about using tables under a separate schema see Creating input EXPLAIN tables under a separate schema. You can find sample CREATE statements for the tables and associated indexes in members DSNTESC and DSNTESH of the prefix.SDSNSAMP library.
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.
All object names are UPPER CASE in the query text of the SQL statements.
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.

About this task

Statement-level access paths use matching of the statement text to apply the specified access path to all instances of a statement within one of the following scopes:

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

Procedure

To create statement-level access paths:

INSERT rows into the DSN_USERQUERY_TABLE table.
1. Insert values in the following columns to specify the SQL statement and context in which to apply the access path:
  QUERYNO
  
  Insert the value that correlates to the value of the QUERYNO column of existing PLAN_TABLE rows that describe the access path that you want to enforce.
  
  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 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.
  
  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.
  For example, you might execute either of the following statements to populate DSN_USERQUERY_TABLE.
  - For static SQL statements, you might retrieve values from the SYSIBM.SYSPACKSTMT catalog table and insert the values by executing a statement like the following INSERT statement:
```
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 
)   
SELECT 1111111, 'MYSCHEMA_1', 1, 
STATEMENT, 
'','',
COLLID, NAME, VERSION, 
'', '', 
-1, '', -1, ''
FROM SYSIBM.SYSPACKSTMT
WHERE COLLID = ' MYCOLLID_1'
AND NAME = 'MYPACKAGE_1'
AND VERSION = 'MYVERSION_1'
AND STMTNO = 12;
```
    When validated, the result specifies that DB2 uses the access path that is described by the PLAN_TABLE row that contains the QUERYNO value of 1111111 for instances of the specified statement that are issued under the specified package version. The 1 value for HINT_SCOPE indicates that the hint applies only to instances of the statement that are issued by the specified package.
  - If the statement text and other information are not available from the SYSIBM.SYSPACKSTMT catalog table, you might issue an INSERT statement that specifies the values explicitly.
Populate PLAN_TABLE for the SQL statement. You can populate this table by either manually inserting one or more rows or issuing an EXPLAIN statement.
Issue a BIND QUERY command. You must omit the LOOKUP option or specify LOOKUP(NO). 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.SYSQUERYPLAN
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.

If DB2 uses all of the access paths that you specified, it returns SQLCODE +394 from the PREPARE of the EXPLAIN statement and from the PREPARE of SQL statements that use the specified access paths. If any of your the specified access paths are invalid, or if any duplicates were found, DB2 issues SQLCODE +395. Start of change You can suppress SQLCODES +394 and +395 for dynamic SQL statements by setting the value of the SUPPRESS_HINT_SQLCODE_DYN subsystem parameter. End of change

What to do next

Consider taking the following actions:

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.
  - 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.
  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.
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.