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LIMIT ROWS EXAMINED prematurely triggers during optimization

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1. LIMIT ROWS EXAMINED clause allows to abort a query if a certain limit
of rows involved in processing is exceeded. This limitation should be only
enforced during the execution phase and not the optimization.

2. Unions are often executed using so-called "fake_select_lex" which
collects the final result from union parts. During that finalization
LIMIT ROWS EXAMINED must be ignored to avoid producing a potentially
incomplete result. There is a workaround at `st_select_lex_unit::exec()`
which deactivates the limit before fake_select_lex processing, and
re-activates it when the processing is finished. However, this re-activation
does not take into account whether the limit was active before the start
of fake_select_lex processing.

3. `st_select_lex_unit::exec()` can be invoked during the optimization
phase for evaluation of constant conditions with unions. At that time
the limit trigger is not activated. Given that the re-activation
mentioned above does not respect the previous state of the limit trigger,
a premature activation of the limit may happen.

This commit fixes that behavior by storing the state of the trigger
before its deactivation at `st_select_lex_unit::exec()` and re-activating
it only if the limit was active before.

4. This commit also removes the call to `thd->lex->set_rows_examined()`
from `st_select_lex_unit::exec_recursive()` which was probably copied
from `st_select_lex_unit::exec()`. But in the context of
 `exec_recursive()` the call does not make sense as there is no
previous deactivation of the limit as at `exec()`.

Reviewed by: Dave Gosselin, Sergei Petrunia
This commit is contained in:
Oleg Smirnov
2025-09-18 21:55:17 +03:00
parent f64e3fe3f6
commit a347f7a158
4 changed files with 33 additions and 8 deletions
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11
mysql-test/main/limit_rows_examined.result
View File

@@ -938,3 +938,14 @@ Warnings:
Warning 1931 Query execution was interrupted. The query exceeded LIMIT ROWS EXAMINED 100. The query result may be incomplete
DROP TABLE t1, t2;
# End of 10.5 tests
#
# MDEV-22241: Assertion `0' failed in Protocol::end_statement after query with LIMIT ROWS EXAMINED
#
CREATE TABLE t1 (a int);
INSERT INTO t1 VALUES (1);
SELECT 1 FROM t1
WHERE (1 IN (SELECT 8 UNION SELECT 5)) OR t1.a = 2
LIMIT ROWS EXAMINED 1;
1
DROP TABLE t1;
# End of 10.11 tests

15
mysql-test/main/limit_rows_examined.test
View File

@@ -658,3 +658,18 @@ SELECT COUNT(*) FROM t1 JOIN t2 ON (b = a) UNION DISTINCT SELECT COUNT(*) FROM t
DROP TABLE t1, t2;
--echo # End of 10.5 tests
--echo #
--echo # MDEV-22241: Assertion `0' failed in Protocol::end_statement after query with LIMIT ROWS EXAMINED
--echo #
CREATE TABLE t1 (a int);
INSERT INTO t1 VALUES (1);
SELECT 1 FROM t1
WHERE (1 IN (SELECT 8 UNION SELECT 5)) OR t1.a = 2
LIMIT ROWS EXAMINED 1;
DROP TABLE t1;
--echo # End of 10.11 tests

4
sql/sql_lex.h
View File

@@ -3604,8 +3604,6 @@ public:
{
if (limit_rows_examined)
limit_rows_examined_cnt= limit_rows_examined->val_uint();
else
limit_rows_examined_cnt= ULONGLONG_MAX; // Unreachable value
}
/**
@@ -3617,7 +3615,7 @@ public:
*/
bool deactivate_limit_rows_examined()
{
bool was_activated= (limit_rows_examined_cnt == ULONGLONG_MAX);
bool was_activated= (limit_rows_examined_cnt != ULONGLONG_MAX);
limit_rows_examined_cnt= ULONGLONG_MAX; // Unreachable value
return was_activated;
}

11
sql/sql_union.cc
View File

@@ -2340,14 +2340,15 @@ bool st_select_lex_unit::exec()
DBUG_EXECUTE_IF("show_explain_probe_union_read",
dbug_serve_apcs(thd, 1););
bool limit_rows_was_activated;
{
List<Item_func_match> empty_list;
empty_list.empty();
/*
Deactivate LIMIT ROWS EXAMINED in order to produce the possibly incomplete
result of the UNION without interruption due to exceeding the limit.
Deactivate LIMIT ROWS EXAMINED to avoid producing potentially incomplete
result of the UNION due to exceeding of the limit.
*/
thd->lex->deactivate_limit_rows_examined();
limit_rows_was_activated= thd->lex->deactivate_limit_rows_examined();
// Check if EOM
if (fake_select_lex != NULL && likely(!thd->is_fatal_error))
@@ -2447,7 +2448,8 @@ bool st_select_lex_unit::exec()
}
thd->lex->current_select= lex_select_save;
err:
thd->lex->activate_limit_rows_examined();
if (limit_rows_was_activated)
thd->lex->activate_limit_rows_examined();
if (likely(!saved_error))
thd->inc_examined_row_count(examined_rows);
DBUG_RETURN(saved_error);
@@ -2588,7 +2590,6 @@ bool st_select_lex_unit::exec_recursive()
thd->lex->current_select= lex_select_save;
err:
thd->lex->activate_limit_rows_examined();
DBUG_RETURN(saved_error);
}