MySQL 5.7 Release Notes
One means of control over optimizer strategies is to set the
optimizer_switch system
variable (see Section 8.9.2, “Switchable Optimizations”).
Changes to this variable affect execution of all subsequent
queries; to affect one query differently from another, it is
necessary to change
optimizer_switch before each
one.
another way to control the optimizer is by using optimizer
hints, which can be specified within individual statements.
Because optimizer hints apply on a per-statement basis, they
provide finer control over statement execution plans than can be
achieved using
optimizer_switch. For example,
you can enable an optimization for one table in a statement and
disable the optimization for a different table. Hints within a
statement take precedence over
optimizer_switch flags.
Examples:
SELECT /*+ NO_RANGE_OPTIMIZATION(t3 PRIMARY, f2_idx) */ f1
FROM t3 WHERE f1 > 30 AND f1 < 33;
SELECT /*+ BKA(t1) NO_BKA(t2) */ * FROM t1 INNER JOIN t2 WHERE ...;
SELECT /*+ NO_ICP(t1, t2) */ * FROM t1 INNER JOIN t2 WHERE ...;
SELECT /*+ SEMIJOIN(FIRSTMATCH, LOOSESCAN) */ * FROM t1 ...;
EXPLAIN SELECT /*+ NO_ICP(t1) */ * FROM t1 WHERE ...;
Note
The mysql client by default strips comments
from SQL statements sent to the server (including optimizer
hints) until MySQL 5.7.7, when it was changed to pass
optimizer hints to the server. To ensure that optimizer hints
are not stripped if you are using an older version of the
mysql client with a version of the server
that understands optimizer hints, invoke
mysql with the
--comments option.
Optimizer hints, described here, differ from index hints, described in Section 8.9.4, “Index Hints”. Optimizer and index hints may be used separately or together.
Optimizer hints apply at different scope levels:
Global: The hint affects the entire statement
Query block: The hint affects a particular query block within a statement
Table-level: The hint affects a particular table within a query block
Index-level: The hint affects a particular index within a table
The following table summarizes the available optimizer hints, the optimizer strategies they affect, and the scope or scopes at which they apply. More details are given later.
Table 8.2 Optimizer Hints Available
| Hint Name | Description | Applicable Scopes |
|---|---|---|
BKA,
NO_BKA |
Affects Batched Key Access join processing | Query block, table |
BNL,
NO_BNL |
Affects Block Nested-Loop join processing | Query block, table |
MAX_EXECUTION_TIME |
Limits statement execution time | Global |
MRR,
NO_MRR |
Affects Multi-Range Read optimization | Table, index |
NO_ICP |
Affects Index Condition Pushdown optimization | Table, index |
NO_RANGE_OPTIMIZATION |
Affects range optimization | Table, index |
QB_NAME |
Assigns name to query block | Query block |
SEMIJOIN,
NO_SEMIJOIN |
semijoin strategies | Query block |
SUBQUERY |
Affects materialization,
IN-to-EXISTS
subquery stratgies |
Query block |
Disabling an optimization prevents the optimizer from using it. Enabling an optimization means the optimizer is free to use the strategy if it applies to statement execution, not that the optimizer necessarily uses it.
MySQL supports comments in SQL statements as described in
Section 9.6, “Comments”. Optimizer hints must be specified
within /*+ ... */ comments. That is,
optimizer hints use a variant of /* ... */
C-style comment syntax, with a + character
following the /* comment opening sequence.
Examples:
/*+ BKA(t1) */
/*+ BNL(t1, t2) */
/*+ NO_RANGE_OPTIMIZATION(t4 PRIMARY) */
/*+ QB_NAME(qb2) */
Whitespace is permitted after the +
character.
The parser recognizes optimizer hint comments after the
initial keyword of SELECT,
UPDATE,
INSERT,
REPLACE, and
DELETE statements. Hints are
permitted in these contexts:
At the beginning of query and data change statements:
SELECT /*+ ... */ ... INSERT /*+ ... */ ... REPLACE /*+ ... */ ... UPDATE /*+ ... */ ... DELETE /*+ ... */ ...At the beginning of query blocks:
(SELECT /*+ ... */ ... ) (SELECT ... ) UNION (SELECT /*+ ... */ ... ) (SELECT /*+ ... */ ... ) UNION (SELECT /*+ ... */ ... ) UPDATE ... WHERE x IN (SELECT /*+ ... */ ...) INSERT ... SELECT /*+ ... */ ...In hintable statements prefaced by
EXPLAIN. For example:EXPLAIN SELECT /*+ ... */ ... EXPLAIN UPDATE ... WHERE x IN (SELECT /*+ ... */ ...)The implication is that you can use
EXPLAINto see how optimizer hints affect execution plans. UseSHOW WARNINGSimmediately afterEXPLAINto see how hints are used. The extendedEXPLAINoutput displayed by a followingSHOW WARNINGSindicates which hints were used. Ignored hints are not displayed.
A hint comment may contain multiple hints, but a query block cannot contain multiple hint comments. This is valid:
SELECT /*+ BNL(t1) BKA(t2) */ ...But this is invalid:
SELECT /*+ BNL(t1) */ /* BKA(t2) */ ...When a hint comment contains multiple hints, the possibility of duplicates and conflicts exists. The following general guidelines apply. For specific hint types, additional rules may apply, as indicated in the hint descriptions.
Duplicate hints: For a hint such as
/*+ MRR(idx1) MRR(idx1) */, MySQL uses the first hint and issues a warning about the duplicate hint.Conflicting hints: For a hint such as
/*+ MRR(idx1) NO_MRR(idx1) */, MySQL uses the first hint and issues a warning about the second conflicting hint.
Query block names are identifiers and follow the usual rules about what names are valid and how to quote them (see Section 9.2, “Schema Object Names”).
Hint names, query block names, and strategy names are not case-sensitive. References to table and index names follow the usual identifier case sensitivity rules (see Section 9.2.3, “Identifier Case Sensitivity”).
Table-level hints affect use of the Block Nested-Loop (BNL) and Batched Key Access (BKA) join-processing algorithms (see Section 8.2.1.11, “Block Nested-Loop and Batched Key Access Joins”). These hint types apply to specific tables, or all tables in a query block.
Syntax of table-level hints:
hint_name([@query_block_name] [tbl_name [, tbl_name] ...])
hint_name([tbl_name@query_block_name [, tbl_name@query_block_name] ...])The syntax refers to these terms:
hint_name: These hint names are permitted:NoteTo use a BNL or BKA hint to enable join buffering for any inner table of an outer join, join buffering must be enabled for all inner tables of the outer join.
tbl_name: The name of a table used in the statement. The hint applies to all tables that it names. If the hint names no tables, it applies to all tables of the query block in which it occurs.If a table has an alias, hints must refer to the alias, not the table name.
Table names in hints cannot be qualified with schema names.
query_block_name: The query block to which the hint applies. If the hint includes no leading@, the hint applies to the query block in which it occurs. Forquery_block_nametbl_name@query_block_name
Examples:
SELECT /*+ NO_BKA(t1, t2) */ t1.* FROM t1 INNER JOIN t2 INNER JOIN t3;
SELECT /*+ NO_BNL() BKA(t1) */ t1.* FROM t1 INNER JOIN t2 INNER JOIN t3;A table-level hint applies to tables that receive records from previous tables, not sender tables. Consider this statement:
SELECT /*+ BNL(t2) */ FROM t1, t2;
If the optimizer chooses to process t1
first, it applies a Block Nested-Loop join to
t2 by buffering the rows from
t1 before starting to read from
t2. If the optimizer instead chooses to
process t2 first, the hint has no effect
because t2 is a sender table.
Index-level hints affect which index-processing strategies the optimizer uses for particular tables or indexes. These hint types affect use of Index Condition Pushdown (ICP), Multi-Range Read (MRR), and range optimizations (see Section 8.2.1, “Optimizing SELECT Statements”).
Syntax of index-level hints:
hint_name([@query_block_name] tbl_name [index_name [, index_name] ...])
hint_name(tbl_name@query_block_name [index_name [, index_name] ...])The syntax refers to these terms:
hint_name: These hint names are permitted:MRR,NO_MRR: Enable or disable MRR for the specified table or indexes. MRR hints apply only toInnoDBandMyISAMtables.NO_ICP: Disable ICP for the specified table or indexes. By default, ICP is a candidate optimization strategy, so there is no hint for enabling it.NO_RANGE_OPTIMIZATION: Disable index range access for the specified table or indexes. This hint also disables Index Merge and Loose Index Scan for the table or indexes. By default, range access is a candidate optimization strategy, so there is no hint for enabling it.This hint may be useful when the number of ranges may be high and range optimization would require many resources.
tbl_name: The table to which the hint applies.index_name: The name of an index in the named table. The hint applies to all indexes that it names. If the hint names no indexes, it applies to all indexes in the table.To refer to a primary key, use the name
PRIMARY. To see the index names for a table, useSHOW INDEX.query_block_name: The query block to which the hint applies. If the hint includes no leading@, the hint applies to the query block in which it occurs. Forquery_block_nametbl_name@query_block_name
Examples:
SELECT /*+ MRR(t1) */ * FROM t1 WHERE f2 <= 3 AND 3 <= f3;
SELECT /*+ NO_RANGE_OPTIMIZATION(t3 PRIMARY, f2_idx) */ f1
FROM t3 WHERE f1 > 30 AND f1 < 33;
INSERT INTO t3(f1, f2, f3)
(SELECT /*+ NO_ICP(t2) */ t2.f1, t2.f2, t2.f3 FROM t1,t2
WHERE t1.f1=t2.f1 AND t2.f2 BETWEEN t1.f1
AND t1.f2 AND t2.f2 + 1 >= t1.f1 + 1);
Subquery hints affect whether to use semijoin transformations
and which semijoin strategies to permit, and, when semijoins
are not used, whether to use subquery materialization or
IN-to-EXISTS
transformations. For more information about these
optimizations, see Section 8.2.2, “Optimizing Subqueries, Derived Tables, and View References”.
Syntax of hints that affect semijoin strategies:
hint_name([@query_block_name] [strategy [, strategy] ...])The syntax refers to these terms:
hint_name: These hint names are permitted:SEMIJOIN,NO_SEMIJOIN: Enable or disable the named semijoin strategies.
strategy: A semijoin strategy to be enabled or disabled. These strategy names are permitted:DUPSWEEDOUT,FIRSTMATCH,LOOSESCAN,MATERIALIZATION.For
SEMIJOINhints, if no strategies are named, semijoin is used if possible based on the strategies enabled according to theoptimizer_switchsystem variable. If strategies are named but inapplicable for the statement,DUPSWEEDOUTis used.For
NO_SEMIJOINhints, if no strategies are named, semijoin is not used. If strategies are named that rule out all applicable strategies for the statement,DUPSWEEDOUTis used.
If one subquery is nested within another and both are merged
into a semijoin of an outer query, any specification of
semijoin strategies for the innermost query are ignored.
SEMIJOIN and
NO_SEMIJOIN hints can still
be used to enable or disable semijoin transformations for such
nested subqueries.
If DUPSWEEDOUT is disabled, on occasion the
optimizer may generate a query plan that is far from optimal.
This occurs due to heuristic pruning during greedy search,
which can be avoided by setting
optimizer_prune_level=0.
Examples:
SELECT /*+ NO_SEMIJOIN(@subq1 FIRSTMATCH, LOOSESCAN) */ * FROM t2
WHERE t2.a IN (SELECT /*+ QB_NAME(subq1) */ a FROM t3);
SELECT /*+ SEMIJOIN(@subq1 MATERIALIZATION, DUPSWEEDOUT) */ * FROM t2
WHERE t2.a IN (SELECT /*+ QB_NAME(subq1) */ a FROM t3);
Syntax of hints that affect whether to use subquery
materialization or
IN-to-EXISTS
transformations:
SUBQUERY([@query_block_name] strategy)
The hint name is always
SUBQUERY.
For SUBQUERY hints, these
strategy values are permitted:
INTOEXISTS,
MATERIALIZATION.
Examples:
SELECT id, a IN (SELECT /*+ SUBQUERY(MATERIALIZATION) */ a FROM t1) FROM t2;
SELECT * FROM t2 WHERE t2.a IN (SELECT /*+ SUBQUERY(INTOEXISTS) */ a FROM t1);
For semijoin and SUBQUERY
hints, a leading
@
specifies the query block to which the hint applies. If the
hint includes no leading
query_block_name@,
the hint applies to the query block in which it occurs. To
assign a name to a query block, see
Optimizer Hints for Naming Query Blocks.
query_block_name
If a hint comment contains multiple subquery hints, the first is used. If there are other following hints of that type, they produce a warning. Following hints of other types are silently ignored.
The MAX_EXECUTION_TIME hint
is permitted only for SELECT
statements. It places a limit N (a
timeout value in milliseconds) on how long a statement is
permitted to execute before the server terminates it:
MAX_EXECUTION_TIME(N)Example with a timeout of 1 second (1000 milliseconds):
SELECT /*+ MAX_EXECUTION_TIME(1000) */ * FROM t1 INNER JOIN t2 WHERE ...
The
MAX_EXECUTION_TIME(
hint sets a statement execution timeout of
N)N milliseconds. If this option is
absent or N is 0, the statement
timeout established by the
max_execution_time system
variable applies.
The MAX_EXECUTION_TIME hint
is applicable as follows:
For statements with multiple
SELECTkeywords, such as unions or statements with subqueries,MAX_EXECUTION_TIMEapplies to the entire statement and must appear after the firstSELECT.It applies to read-only
SELECTstatements. Statements that are not read only are those that invoke a stored function that modifies data as a side effect.It does not apply to
SELECTstatements in stored programs and is ignored.
Table-level, index-level, and subquery optimizer hints permit
specific query blocks to be named as part of their argument
syntax. To create these names, use the
QB_NAME hint, which assigns
a name to the query block in which it occurs:
QB_NAME(name)
QB_NAME hints can be used to
make explicit in a clear way which query blocks other hints
apply to. They also permit all non-query block name hints to
be specified within a single hint comment for easier
understanding of complex statements. Consider the following
statement:
SELECT ...
FROM (SELECT ...
FROM (SELECT ... FROM ...)) ...
QB_NAME hints assign names
to query blocks in the statement:
SELECT /*+ QB_NAME(qb1) */ ...
FROM (SELECT /*+ QB_NAME(qb2) */ ...
FROM (SELECT /*+ QB_NAME(qb3) */ ... FROM ...)) ...Then other hints can use those names to refer to the appropriate query blocks:
SELECT /*+ QB_NAME(qb1) MRR(@qb1 t1) BKA(@qb2) NO_MRR(@qb3t1 idx1, id2) */ ...
FROM (SELECT /*+ QB_NAME(qb2) */ ...
FROM (SELECT /*+ QB_NAME(qb3) */ ... FROM ...)) ...The resulting effect is as follows:
MRR(@qb1 t1)applies to tablet1in query blockqb1.BKA(@qb2)applies to query blockqb2.NO_MRR(@qb3 t1 idx1, id2)applies to indexesidx1andidx2in tablet1in query blockqb3.
Query block names are identifiers and follow the usual rules about what names are valid and how to quote them (see Section 9.2, “Schema Object Names”). For example, a query block name that contains spaces must be quoted, which can be done using backticks:
SELECT /*+ BKA(@`my hint name`) */ ...
FROM (SELECT /*+ QB_NAME(`my hint name`) */ ...) ...
If the ANSI_QUOTES SQL mode
is enabled, it is also possible to quote query block names
within double quotation marks:
SELECT /*+ BKA(@"my hint name") */ ...
FROM (SELECT /*+ QB_NAME("my hint name") */ ...) ...