MDEV-39349 NO_INDEX hint using QB_NAME doesn't work inside VIEWs#4996
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MDEV-39349 NO_INDEX hint using QB_NAME doesn't work inside VIEWs#4996
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Refine the logic of storing hints inside view definitions. The same procedures are used for printing hints in both EXPLAIN EXTENDED of regular statements and when storing view definitions. For regular statements all hints (except QB_NAME) are printed into the topmost SELECT with attachment of target query block names. If some query blocks do not have explicit names set with QB_NAME hints, implicit names `select#1`, `select#2` are attached. This approach does not work views, because view definitions do not have such implicit numbering of query blocks. The solution is to print hints related to a particular query block right into this query block instead of printing them into the topmost SELECT.
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There two variants of syntax supported for join order hints: - hint([@qb_name] t1, t2, t3,...) - hint (t1[@qb1], t2[@qb2],...) However, when creating views, internal procedures may produce a mixed variant of both: - hint (@qb_name t1, t2@qb1, t3...) This commit introduces support for this third variant of syntax for join order hints, so it is supported for both regular statements and for views
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Looks good!
Consider making this change in sql_select.cc, to remove the duplicate conditional branches:
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -32620,13 +32620,9 @@ void st_select_lex::print_hints(THD *thd, String *str)
*/
parent_lex->opt_hints_global->print(thd, str);
}
- else if (opt_hints_qb)
- {
- // For views print hints related to a QB right into the QB they target
- opt_hints_qb->print(thd, str);
- }
}
- else if (is_view && opt_hints_qb)
+
+ if (is_view && opt_hints_qb)
{
// For views print hints related to a QB right into the QB they target
opt_hints_qb->print(thd, str);
| # End Examples from QA | ||
| # End of 12.2 tests | ||
| # | ||
| # NO_INDEX hint using QB_NAME doesn't work inside VIEWs |
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This is a little confusing because the tests below show that it does work, see next comment...
| CREATE VIEW v1 AS SELECT /*+ QB_NAME(foo) NO_INDEX(t1@foo) */ a, b FROM t1 WHERE a<3 LIMIT 10; | ||
| SHOW CREATE VIEW v1; | ||
| View Create View character_set_client collation_connection | ||
| v1 CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v1` AS select /*+ QB_NAME(`foo`) NO_INDEX(`t1`@`foo`) */ `t1`.`a` AS `a`,`t1`.`b` AS `b` from `t1` where `t1`.`a` < 3 limit 10 utf8mb3 utf8mb3_uca1400_ai_ci |
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... here we see the hint is in the SHOW CREATE VIEW output and the query plan shows full scan ...
| 1 PRIMARY <derived2> ALL NULL NULL NULL NULL 10 100.00 | ||
| 2 DERIVED t1 ALL NULL NULL NULL NULL 1000 100.00 Using where | ||
| Warnings: | ||
| Note 1003 /* select#1 */ select `v1`.`a` AS `a`,`v1`.`b` AS `b` from `test`.`v1` |
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... so is the issue just that the hint doesn't appear here? Is it because the VIEW was merged and so we can't show the hint anymore (even though it applied)?
(If yes, then consider adding a "Limitations" section to the git commit message to document this detail)
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Refine the logic of storing hints inside view definitions.
The same procedures are used for printing hints in both
EXPLAIN EXTENDED of regular statements and when storing view
definitions. For regular statements all hints (except QB_NAME)
are printed into the topmost SELECT with attachment of target
query block names. If some query blocks do not have explicit
names set with QB_NAME hints, implicit names
select#1,select#2are attached.
This approach does not work views, because view definitions do not
have such implicit numbering of query blocks. The solution is
to print hints related to a particular query block right
into this query block instead of printing them into
the topmost SELECT.