revision-id: 677f1ef6f00793b3ad2a42b4e6f0fcbb7cd0e39d (mariadb-10.2.31-950-g677f1ef) parent(s): e607f3398c69147299884d3814cf063d2e7516ce author: Igor Babaev committer: Igor Babaev timestamp: 2021-05-14 16:43:36 -0700 message: MDEV-25682 Explain shows an execution plan different from actually executed 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> --- mysql-test/r/order_by.result | 25 +++++++++++++++++++++++++ mysql-test/t/order_by.test | 16 ++++++++++++++++ sql/sql_select.cc | 2 +- 3 files changed, 42 insertions(+), 1 deletion(-) diff --git a/mysql-test/r/order_by.result b/mysql-test/r/order_by.result index b144101..39b4e25 100644 --- a/mysql-test/r/order_by.result +++ b/mysql-test/r/order_by.result @@ -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 diff --git a/mysql-test/t/order_by.test b/mysql-test/t/order_by.test index 36c25ed..4e50fc5 100644 --- a/mysql-test/t/order_by.test +++ b/mysql-test/t/order_by.test @@ -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 diff --git a/sql/sql_select.cc b/sql/sql_select.cc index b85bd31..ce70620 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -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) {