revision-id: d3c6620b58215664cbc2568243c164fa1b89a2c0 (mariadb-10.3.31-70-gd3c6620b582) parent(s): 9962cda52722b77c2a7e0314bbaa2e4f963f55c1 author: Sergei Petrunia committer: Sergei Petrunia timestamp: 2022-01-19 22:45:11 +0300 message: MDEV-26249: Crash in Explain_node::print_explain_for_children with slow query log The problem affected queries in form: SELECT FROM (SELECT where Split Materialized is applicable) WHERE 1=0 The problem was caused by this: - The select in derived table uses two-phase optimization (due to a possible LATERAL DERIVED). - The primary select has "Impossible where" and so it short-cuts its optimization. - The optimization for the SELECT in the derived table is never finished, and EXPLAIN data structure has a dangling pointer to select #2. Fixed with this: if the select has "Impossible where", do not add links to derived table subselects into the EXPLAIN data structure. We are not going to execute those anyway. --- mysql-test/main/explain_innodb.result | 20 ++++++++++++++++++++ mysql-test/main/explain_innodb.test | 20 ++++++++++++++++++++ sql/sql_select.cc | 7 +++++++ 3 files changed, 47 insertions(+) diff --git a/mysql-test/main/explain_innodb.result b/mysql-test/main/explain_innodb.result new file mode 100644 index 00000000000..fe51e45e35d --- /dev/null +++ b/mysql-test/main/explain_innodb.result @@ -0,0 +1,20 @@ +# +# MDEV-26249: Crash in in Explain_node::print_explain_for_children while writing to the slow query log +# +set @sql_tmp=@@slow_query_log; +SET GLOBAL slow_query_log = 1; +SET long_query_time = 0.000000; +SET log_slow_verbosity = 'explain'; +CREATE TABLE t1 ( id varchar(50), KEY (id)) engine=innodb; +SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0; +id +select 1; +1 +1 +explain +SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0; +id select_type table type possible_keys key key_len ref rows Extra +1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE +2 DERIVED t1 index NULL id 53 NULL 1 Using index +SET GLOBAL slow_query_log = @sql_tmp; +drop table t1; diff --git a/mysql-test/main/explain_innodb.test b/mysql-test/main/explain_innodb.test new file mode 100644 index 00000000000..2c29a6e26da --- /dev/null +++ b/mysql-test/main/explain_innodb.test @@ -0,0 +1,20 @@ +--echo # +--echo # MDEV-26249: Crash in in Explain_node::print_explain_for_children while writing to the slow query log +--echo # + +--source include/have_innodb.inc + +set @sql_tmp=@@slow_query_log; +SET GLOBAL slow_query_log = 1; +SET long_query_time = 0.000000; +SET log_slow_verbosity = 'explain'; + +CREATE TABLE t1 ( id varchar(50), KEY (id)) engine=innodb; +SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0; +select 1; + +explain +SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0; + +SET GLOBAL slow_query_log = @sql_tmp; +drop table t1; diff --git a/sql/sql_select.cc b/sql/sql_select.cc index fe02e7b44e4..0de7d940479 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -2739,6 +2739,13 @@ int JOIN::optimize_stage2() */ if (optimize_unflattened_subqueries()) DBUG_RETURN(1); + + /* + We also need to finish optimization of derived tables that we are not using. + Just to show them in [SHOW]EXPLAIN. + */ + if (select_lex->handle_derived(thd->lex, DT_OPTIMIZE)) + DBUG_RETURN(1); error= 0; derived_exit: