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MDEV-25682 Explain shows an execution plan different from actually executed

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If a select query contained an ORDER BY clause that followed a LIMIT clause
or an ORDER BY clause or ORDER BY with LIMIT the EXPLAIN output for the
query showed an execution plan different from that was actually executed.

Approved by Roman Nozdrin <roman.nozdrin@mariadb.com>
This commit is contained in:
Igor Babaev
2021-05-14 16:43:36 -07:00
parent e607f3398c
commit 677f1ef6f0
3 changed files with 42 additions and 1 deletions
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25
mysql-test/r/order_by.result
View File

@ -3460,4 +3460,29 @@ SET max_length_for_sort_data=@save_max_length_for_sort_data;
SET max_sort_length= @save_max_sort_length;
SET sql_select_limit= @save_sql_select_limit;
DROP TABLE t1;
#
# MDEV-25682: EXPLAIN for SELECT with ORDER BY after [ORDER BY] LIMIT
#
create table t1 (a int);
insert into t1 values (3), (7), (1);
explain (select a from t1 limit 2) order by a desc;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3
NULL UNION RESULT <union1> ALL NULL NULL NULL NULL NULL Using filesort
(select a from t1 limit 2) order by a desc;
a
7
3
create table t2 (a int, b int);
insert into t2 values (3,70), (7,10), (1,40), (4,30);
explain (select b,a from t2 order by a limit 3) order by b desc;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 4 Using filesort
NULL UNION RESULT <union1> ALL NULL NULL NULL NULL NULL Using filesort
(select b,a from t2 order by a limit 3) order by b desc;
b a
70 3
40 1
30 4
drop table t1,t2;
# End of 10.2 tests

16
mysql-test/t/order_by.test
View File

@ -2293,4 +2293,20 @@ SET max_sort_length= @save_max_sort_length;
SET sql_select_limit= @save_sql_select_limit;
DROP TABLE t1;
--echo #
--echo # MDEV-25682: EXPLAIN for SELECT with ORDER BY after [ORDER BY] LIMIT
--echo #
create table t1 (a int);
insert into t1 values (3), (7), (1);
explain (select a from t1 limit 2) order by a desc;
(select a from t1 limit 2) order by a desc;
create table t2 (a int, b int);
insert into t2 values (3,70), (7,10), (1,40), (4,30);
explain (select b,a from t2 order by a limit 3) order by b desc;
(select b,a from t2 order by a limit 3) order by b desc;
drop table t1,t2;
--echo # End of 10.2 tests

2
sql/sql_select.cc
View File

@ -25332,7 +25332,7 @@ bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
sl->options|= SELECT_DESCRIBE;
}
if (unit->is_union())
if (unit->is_union() || unit->fake_select_lex)
{
if (unit->union_needs_tmp_table() && unit->fake_select_lex)
{