MySQL 5.7 Release Notes
To enable or disable the plugin, enable or disable the
rewriter_enabled system
variable. By default, the Rewriter plugin is
enabled when you install it (see
Section 5.5.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”).
To set the initial plugin state explicitly, you can set the
variable at server startup. For example, to enable the plugin in
an option file, use these lines:
[mysqld]
rewriter_enabled=ONIt is also possible to enable or disable the plugin at runtime:
SET GLOBAL rewriter_enabled = ON;
SET GLOBAL rewriter_enabled = OFF;
Assuming that the Rewriter plugin is enabled,
it examines and possibly modifies each
SELECT statement received by the
server. The plugin determines whether to rewrite statements
based on its in-memory cache of rewriting rules, which are
loaded from the rewrite_rules table in the
query_rewrite database.
To add rules for the Rewriter plugin, add
rows to the rewrite_rules table, then
invoke the flush_rewrite_rules() stored
procedure to load the rules from the table into the plugin.
The following example creates a simple rule to match
statements that select a single literal value:
INSERT INTO query_rewrite.rewrite_rules (pattern, replacement)
VALUES('SELECT ?', 'SELECT ? + 1');The resulting table contents look like this:
mysql> SELECT * FROM query_rewrite.rewrite_rules\G
*************************** 1. row ***************************
id: 1
pattern: SELECT ?
pattern_database: NULL
replacement: SELECT ? + 1
enabled: YES
message: NULL
pattern_digest: NULL
normalized_pattern: NULL
The rule specifies a pattern template indicating which
SELECT statements to match, and
a replacement template indicating how to rewrite matching
statements. However, adding the rule to the
rewrite_rules table is not sufficient to
cause the Rewriter plugin to use the rule.
You must invoke flush_rewrite_rules() to
load the table contents into the plugin in-memory cache:
mysql> CALL query_rewrite.flush_rewrite_rules();
Tip
If your rewrite rules seem not to be working properly, make
sure that you have reloaded the rules table by calling
flush_rewrite_rules().
When the plugin reads each rule from the rules table, it
computes a normalized (statement digest) form from the pattern
and a digest hash value, and uses them to update the
normalized_pattern and
pattern_digest columns:
mysql> SELECT * FROM query_rewrite.rewrite_rules\G
*************************** 1. row ***************************
id: 1
pattern: SELECT ?
pattern_database: NULL
replacement: SELECT ? + 1
enabled: YES
message: NULL
pattern_digest: 46b876e64cd5c41009d91c754921f1d4
normalized_pattern: select ?For information about statement digesting, normalized statements, and digest hash values, see Section 25.10, “Performance Schema Statement Digests”.
If a rule cannot be loaded due to some error, calling
flush_rewrite_rules() produces an error:
mysql> CALL query_rewrite.flush_rewrite_rules();
ERROR 1644 (45000): Loading of some rule(s) failed.
When this occurs, the plugin writes an error message to the
message column of the rule row to
communicate the problem. Check the
rewrite_rules table for rows with
non-NULL message column
values to see what problems exist.
Patterns use the same syntax as prepared statements (see
Section 13.5.1, “PREPARE Statement”). Within a pattern template,
? characters act as parameter markers that
match data values. The ? characters should
not be enclosed within quotation marks. Parameter markers can
be used only where data values should appear, and they cannot
be used for SQL keywords, identifiers, functions, and so on.
The plugin parses a statement to identify the literal values
(as defined in Section 9.1, “Literal Values”), so you can put a
parameter marker in place of any literal value.
Like the pattern, the replacement can contain
? characters. For a statement that matches
a pattern template, the plugin rewrites it, replacing
? parameter markers in the replacement
using data values matched by the corresponding markers in the
pattern. The result is a complete statement string. The plugin
asks the server to parse it, and returns the result to the
server as the representation of the rewritten statement.
After adding and loading the rule, check whether rewriting occurs according to whether statements match the rule pattern:
mysql> SELECT PI();
+----------+
| PI() |
+----------+
| 3.141593 |
+----------+
1 row in set (0.01 sec)
mysql> SELECT 10;
+--------+
| 10 + 1 |
+--------+
| 11 |
+--------+
1 row in set, 1 warning (0.00 sec)
No rewriting occurs for the first
SELECT statement, but does for
the second. The second statement illustrates that when the
Rewriter plugin rewrites a statement, it
produces a warning message. To view the message, use
SHOW WARNINGS:
mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
Level: Note
Code: 1105
Message: Query 'SELECT 10' rewritten to 'SELECT 10 + 1' by a query rewrite plugin
To enable or disable an existing rule, modify its
enabled column and reload the table into
the plugin. To disable rule 1:
UPDATE query_rewrite.rewrite_rules SET enabled = 'NO' WHERE id = 1;
CALL query_rewrite.flush_rewrite_rules();This enables you to deactivate a rule without removing it from the table.
To re-enable rule 1:
UPDATE query_rewrite.rewrite_rules SET enabled = 'YES' WHERE id = 1;
CALL query_rewrite.flush_rewrite_rules();
The rewrite_rules table contains a
pattern_database column that
Rewriter uses for matching table names that
are not qualified with a database name:
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_databaseand the table names are identical.
Suppose that a table named appdb.users has
a column named id and that applications are
expected to select rows from the table using a query of one of
these forms, where the second can be used when
appdb is the default database:
SELECT * FROM users WHERE appdb.id = id_value;
SELECT * FROM users WHERE id = id_value;
Suppose also that the id column is renamed
to user_id (perhaps the table must be
modified to add another type of ID and it is necessary to
indicate more specifically what type of ID the
id column represents).
The change means that applications must refer to
user_id rather than id
in the WHERE clause, but old applications
that cannot be updated no longer work properly. The
Rewriter plugin can solve this problem by
matching and rewriting problematic statements. To match the
statement SELECT * FROM appdb.users WHERE id =
and rewrite it as
valueSELECT * FROM appdb.users WHERE user_id =
, you can insert a
row representing a replacement rule into the rewrite rules
table. If you also want to match this
valueSELECT using the unqualified table name, it
is also necessary to add an explicit rule. Using
? as a value placeholder, the two
INSERT statements needed look
like this:
INSERT INTO query_rewrite.rewrite_rules
(pattern, replacement) VALUES(
'SELECT * FROM appdb.users WHERE id = ?',
'SELECT * FROM appdb.users WHERE user_id = ?'
);
INSERT INTO query_rewrite.rewrite_rules
(pattern, replacement, pattern_database) VALUES(
'SELECT * FROM users WHERE id = ?',
'SELECT * FROM users WHERE user_id = ?',
'appdb'
);After adding the two new rules, execute the following statement to cause them to take effect:
CALL query_rewrite.flush_rewrite_rules();
Rewriter uses the first rule to match
statements that use the qualified table name, and the second
to match statements that use the unqualified name. The second
rule works only when appdb is the default
database.
The Rewriter plugin uses statement digests
and digest hash values to match incoming statements against
rewrite rules in stages. The
max_digest_length system variable
determines the size of the buffer used for computing statement
digests. Larger values enable computation of digests that
distinguish longer statements. Smaller values use less memory
but increase the likelihood of longer statements colliding
with the same digest value.
The plugin matches each statement to the rewrite rules as follows:
Compute the statement digest hash value and compare it to the rule digest hash values. This is subject to false positives, but serves as a quick rejection test.
If the statement digest hash value matches any pattern digest hash values, match the normalized (statement digest) form of the statement to the normalized form of the matching rule patterns.
If the normalized statement matches a rule, compare the literal values in the statement and the pattern. A
?character in the pattern matches any literal value in the statement. If the statement prepares aSELECTstatement,?in the pattern also matches?in the statement. Otherwise, corresponding literals must be the same.
If multiple rules match a statement, it is nondeterministic which one the plugin uses to rewrite the statement.
If a pattern contains more markers than the replacement, the
plugin discards excess data values. If a pattern contains
fewer markers than the replacement, it is an error. The plugin
notices this when the rules table is loaded, writes an error
message to the message column of the rule
row to communicate the problem, and sets the
Rewriter_reload_error status
variable to ON.
Prepared statements are rewritten at parse time (that is, when they are prepared), not when they are executed later.
Prepared statements differ from nonprepared statements in that
they may contain ? characters as parameter
markers. To match a ? in a prepared
statement, a Rewriter pattern must contain
? in the same location. Suppose that a
rewrite rule has this pattern:
SELECT ?, 3
The following table shows several prepared
SELECT statements and whether
the rule pattern matches them.
| Prepared Statement | Whether Pattern Matches Statement |
|---|---|
PREPARE s AS 'SELECT 3, 3' |
Yes |
PREPARE s AS 'SELECT ?, 3' |
Yes |
PREPARE s AS 'SELECT 3, ?' |
No |
PREPARE s AS 'SELECT ?, ?' |
No |
The Rewriter plugin makes information
available about its operation by means of several status
variables:
mysql> SHOW GLOBAL STATUS LIKE 'Rewriter%';
+-----------------------------------+-------+
| Variable_name | Value |
+-----------------------------------+-------+
| Rewriter_number_loaded_rules | 1 |
| Rewriter_number_reloads | 5 |
| Rewriter_number_rewritten_queries | 1 |
| Rewriter_reload_error | ON |
+-----------------------------------+-------+For descriptions of these variables, see Section 5.5.4.3.4, “Rewriter Query Rewrite Plugin Status Variables”.
When you load the rules table by calling the
flush_rewrite_rules() stored procedure, if
an error occurs for some rule, the CALL
statement produces an error, and the plugin sets the
Rewriter_reload_error status variable to
ON:
mysql> CALL query_rewrite.flush_rewrite_rules();
ERROR 1644 (45000): Loading of some rule(s) failed.
mysql> SHOW GLOBAL STATUS LIKE 'Rewriter_reload_error';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Rewriter_reload_error | ON |
+-----------------------+-------+
In this case, check the rewrite_rules table
for rows with non-NULL
message column values to see what problems
exist.
When the rewrite_rules table is loaded into
the Rewriter plugin, the plugin interprets
statements using the current global value of the
character_set_client system
variable. If the global
character_set_client value is
changed subsequently, the rules table must be reloaded.
A client must have a session
character_set_client value
identical to what the global value was when the rules table
was loaded or rule matching does not work for that client.