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MDEV-23160: SIGSEGV in Explain_node::print_explain_for_children on UNION SELECT

Browse Source 
and also MDEV-25564, MDEV-18157.

Attempt to produce EXPLAIN output caused a crash in
Explain_node::print_explain_for_children. The cause of this was that an
Explain_node (actually a derived) had a link to child select#N, but
there was no query plan present for select#N.

The query plan wasn't present because the subquery was eliminated.
- Either it was a degenerate subquery like "(SELECT 1)" in MDEV-25564.
- Or it was a subquery in a UNION subquery's ORDER BY clause:
   col IN (SELECT ... UNION
           SELECT ... ORDER BY (SELECT FROM t1))

In such cases, legacy code structure in subquery/union processing code(*)
makes it hard to detect that the subquery was eliminated, so we end up
with EXPLAIN data structures (Explain_node::children) having dangling
links to child subqueries.
Do make the checks and don't follow the dangling links.

(In ideal world, we should not have these dangling links. But fixing
the code (*) would have high risk for the stable versions).
This commit is contained in:
Sergei Petrunia
2022-10-21 12:04:00 +03:00
parent 0c06320ae9
commit 6bc2e93381
3 changed files with 90 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

46
mysql-test/main/explain.result
View File

@ -407,3 +407,49 @@ id select_type table type possible_keys key key_len ref rows Extra
2 SUBQUERY t1 system NULL NULL NULL NULL 1
drop table t1, t2;
# End of 10.1 tests
#
# End of 10.2 test
#
#
# MDEV-25564: Server crashed on running some EXPLAIN statements
#
EXPLAIN (SELECT 1,3) UNION (SELECT 2,1) ORDER BY (SELECT 2);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL No tables used
2 UNION NULL NULL NULL NULL NULL NULL NULL No tables used
NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL Using filesort
Warnings:
Note 1249 Select 3 was reduced during optimization
#
# MDEV-23160: SIGSEGV in Explain_node::print_explain_for_children on UNION SELECT
#
CREATE TABLE t1 (a INT);
INSERT INTO t1 VALUES (1),(2),(3);
EXPLAIN
SELECT *
FROM t1
WHERE
a IN (SELECT a FROM t1
UNION
SELECT a FROM t1 ORDER BY (SELECT a))
UNION
SELECT * FROM t1 ORDER BY (SELECT a);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 3 Using where
2 DEPENDENT SUBQUERY t1 ALL NULL NULL NULL NULL 3 Using where
3 DEPENDENT UNION t1 ALL NULL NULL NULL NULL 3 Using where
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL
5 UNION t1 ALL NULL NULL NULL NULL 3
NULL UNION RESULT <union1,5> ALL NULL NULL NULL NULL NULL Using filesort
6 DEPENDENT SUBQUERY NULL NULL NULL NULL NULL NULL NULL No tables used
drop table t1;
explain
VALUES ( (VALUES (2))) UNION VALUES ( (SELECT 3));
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL No tables used
5 SUBQUERY <derived2> ALL NULL NULL NULL NULL 2
2 DERIVED NULL NULL NULL NULL NULL NULL NULL No tables used
3 UNION NULL NULL NULL NULL NULL NULL NULL No tables used
NULL UNION RESULT <union1,3> ALL NULL NULL NULL NULL NULL
Warnings:
Note 1249 Select 4 was reduced during optimization

31
mysql-test/main/explain.test
View File

@ -333,3 +333,34 @@ explain replace into t2 select 100, (select a from t1);
drop table t1, t2;
--echo # End of 10.1 tests
--echo #
--echo # End of 10.2 test
--echo #
--echo #
--echo # MDEV-25564: Server crashed on running some EXPLAIN statements
--echo #
EXPLAIN (SELECT 1,3) UNION (SELECT 2,1) ORDER BY (SELECT 2);
--echo #
--echo # MDEV-23160: SIGSEGV in Explain_node::print_explain_for_children on UNION SELECT
--echo #
CREATE TABLE t1 (a INT);
INSERT INTO t1 VALUES (1),(2),(3);
EXPLAIN
SELECT *
FROM t1
WHERE
a IN (SELECT a FROM t1
UNION
SELECT a FROM t1 ORDER BY (SELECT a))
UNION
SELECT * FROM t1 ORDER BY (SELECT a);
drop table t1;
explain
VALUES ( (VALUES (2))) UNION VALUES ( (SELECT 3));

15
sql/sql_explain.cc
View File

@ -639,7 +639,11 @@ int Explain_node::print_explain_for_children(Explain_query *query,
for (int i= 0; i < (int) children.elements(); i++)
{
Explain_node *node= query->get_node(children.at(i));
if (node->print_explain(query, output, explain_flags, is_analyze))
/*
Note: node may not be present because for certain kinds of subqueries,
the optimizer is not able to see that they were eliminated.
*/
if (node && node->print_explain(query, output, explain_flags, is_analyze))
return 1;
}
return 0;
@ -683,8 +687,15 @@ void Explain_node::print_explain_json_for_children(Explain_query *query,
for (int i= 0; i < (int) children.elements(); i++)
{
Explain_node *node= query->get_node(children.at(i));
/* Derived tables are printed inside Explain_table_access objects */
/*
Note: node may not be present because for certain kinds of subqueries,
the optimizer is not able to see that they were eliminated.
*/
if (!node)
continue;
/* Derived tables are printed inside Explain_table_access objects */
if (!is_connection_printable_in_json(node->connection_type))
continue;