7.6.4.3 Rewriter Query Rewrite Plugin Reference

The following discussion serves as a reference to these elements associated with the Rewriter query rewrite plugin:

• The Rewriter rules table in the query_rewrite database

• Rewriter procedures and functions

• Rewriter system and status variables

7.6.4.3.1 Rewriter Query Rewrite Plugin Rules Table

The rewrite_rules table in the query_rewrite database provides persistent storage for the rules that the Rewriter plugin uses to decide whether to rewrite statements.

Users communicate with the plugin by modifying the set of rules stored in this table. The plugin communicates information to users by setting the table's message column.

Note

The rules table is loaded into the plugin by the flush_rewrite_rules stored procedure. Unless that procedure has been called following the most recent table modification, the table contents do not necessarily correspond to the set of rules the plugin is using.

The rewrite_rules table has these columns:

• id

  The rule ID. This column is the table primary key. You can use the ID to uniquely identify any rule.

• pattern

  The template that indicates the pattern for statements that the rule matches. Use ? to represent parameter markers that match data values.

• pattern_database

  The database used to match unqualified table names in statements. Qualified table names in statements match qualified names in the pattern if corresponding database and table names are identical. Unqualified table names in statements match unqualified names in the pattern only if the default database is the same as pattern_database and the table names are identical.

• replacement

  The template that indicates how to rewrite statements matching the pattern column value. Use ? to represent parameter markers that match data values. In rewritten statements, the plugin replaces ? parameter markers in replacement using data values matched by the corresponding markers in pattern.

• enabled

  Whether the rule is enabled. Load operations (performed by invoking the flush_rewrite_rules() stored procedure) load the rule from the table into the Rewriter in-memory cache only if this column is YES.

  This column makes it possible to deactivate a rule without removing it: Set the column to a value other than YES and reload the table into the plugin.

• message

  The plugin uses this column for communicating with users. If no error occurs when the rules table is loaded into memory, the plugin sets the message column to NULL. A non-NULL value indicates an error and the column contents are the error message. Errors can occur under these circumstances:

  • Either the pattern or the replacement is an incorrect SQL statement that produces syntax errors.

  • The replacement contains more ? parameter markers than the pattern.

  If a load error occurs, the plugin also sets the Rewriter_reload_error status variable to ON.

• pattern_digest

  This column is used for debugging and diagnostics. If the column exists when the rules table is loaded into memory, the plugin updates it with the pattern digest. This column may be useful if you are trying to determine why some statement fails to be rewritten.

• normalized_pattern

  This column is used for debugging and diagnostics. If the column exists when the rules table is loaded into memory, the plugin updates it with the normalized form of the pattern. This column may be useful if you are trying to determine why some statement fails to be rewritten.

7.6.4.3.2 Rewriter Query Rewrite Plugin Procedures and Functions

Rewriter plugin operation uses a stored procedure that loads the rules table into its in-memory cache, and a helper loadable function. Under normal operation, users invoke only the stored procedure. The function is intended to be invoked by the stored procedure, not directly by users.

• flush_rewrite_rules()

  This stored procedure uses the load_rewrite_rules() function to load the contents of the rewrite_rules table into the Rewriter in-memory cache.

  Calling flush_rewrite_rules() implies COMMIT.

  Invoke this procedure after you modify the rules table to cause the plugin to update its cache from the new table contents. If any errors occur, the plugin sets the message column for the appropriate rule rows in the table and sets the Rewriter_reload_error status variable to ON.

• load_rewrite_rules()

  This function is a helper routine used by the flush_rewrite_rules() stored procedure.

7.6.4.3.3 Rewriter Query Rewrite Plugin System Variables

The Rewriter query rewrite plugin supports the following system variables. These variables are available only if the plugin is installed (see Section 7.6.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”).

• rewriter_enabled

  System Variable	rewriter_enabled
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON
  Valid Values	OFF

  Whether the Rewriter query rewrite plugin is enabled.

• rewriter_enabled_for_threads_without_privilege_checks

  System Variable	rewriter_enabled_for_threads_without_privilege_checks
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON
  Valid Values	OFF

  Whether to apply rewrites for replication threads which execute with privilege checks disabled. If set to OFF, such rewrites are skipped. Requires the SYSTEM_VARIABLES_ADMIN privilege or SUPER privilege to set.

  This variable has no effect if rewriter_enabled is OFF.

• rewriter_verbose

  System Variable	rewriter_verbose
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer

  For internal use.

7.6.4.3.4 Rewriter Query Rewrite Plugin Status Variables

The Rewriter query rewrite plugin supports the following status variables. These variables are available only if the plugin is installed (see Section 7.6.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”).

• Rewriter_number_loaded_rules

  The number of rewrite plugin rewrite rules successfully loaded from the rewrite_rules table into memory for use by the Rewriter plugin.

• Rewriter_number_reloads

  The number of times the rewrite_rules table has been loaded into the in-memory cache used by the Rewriter plugin.

• Rewriter_number_rewritten_queries

  The number of queries rewritten by the Rewriter query rewrite plugin since it was loaded.

• Rewriter_reload_error

  Whether an error occurred the most recent time that the rewrite_rules table was loaded into the in-memory cache used by the Rewriter plugin. If the value is OFF, no error occurred. If the value is ON, an error occurred; check the message column of the rewriter_rules table for error messages.