| Bug #120298 | opt_explain_json.cc:507 assert failure — EXPLAIN FORMAT=JSON on INTERSECT subquery crashes mysqld | ||
|---|---|---|---|
| Submitted: | 20 Apr 8:58 | Modified: | 20 Apr 10:56 |
| Reporter: | Chunling Qin | Email Updates: | |
| Status: | Can't repeat | Impact on me: | |
| Category: | MySQL Server: DML | Severity: | S3 (Non-critical) |
| Version: | 8.0.31-tr-debug | OS: | Any |
| Assigned to: | CPU Architecture: | Any | |
[20 Apr 10:48]
Roy Lyseng
Thank you for the bug report. I am not sure what you are seeing here. With the patch for WL#349 that added support for INTERSECT and EXCEPT, the set_child() function is also extended to add support for CTX_INTERSECT and CTX_EXCEPT. In any case, the problem is not reproducible on later 8.0 releases.
[20 Apr 10:56]
Chunling Qin
mysql> select version(); +-----------------+ | version() | +-----------------+ | 8.0.31-tr-debug | +-----------------+ 1 row in set (0.00 sec) mysql> EXPLAIN FORMAT=JSON SELECT * FROM (SELECT 1 INTERSECT SELECT 2) AS t; ERROR 1235 (42000): This version of MySQL doesn't yet support 'INTERSECT' mysql> SET SESSION debug_enable_extended_set_ops=ON; Query OK, 0 rows affected (0.00 sec) mysql> EXPLAIN FORMAT=JSON SELECT * FROM (SELECT 1 INTERSECT SELECT 2) AS t; ERROR 2013 (HY000): Lost connection to MySQL server during query No connection. Trying to reconnect... ERROR 2003 (HY000): Can't connect to MySQL server on '9.134.129.225:3306' (111) ERROR: Can't connect to the server
Description: Description Executing EXPLAIN FORMAT=JSON on a query that contains an INTERSECT set operation inside a derived table (subquery) triggers an assertion failure in subquery_ctx::set_child(), causing the mysqld process to abort and generate a core dump. The crash occurs because the EXPLAIN FORMAT=JSON infrastructure only expects child context types of CTX_JOIN or CTX_UNION, but the INTERSECT set operation introduces a context type that is neither, hitting the assert(false) guard. Root Cause In sql/opt_explain_json.cc:505-509, the subquery_ctx::set_child() method asserts that any child context must be either CTX_JOIN or CTX_UNION: // sql/opt_explain_json.cc:505-509 void set_child(context *child) override { assert(subquery == nullptr); assert(child->type == CTX_JOIN || child->type == CTX_UNION); // <-- crash subquery = child; } When debug_enable_extended_set_ops=ON is enabled and an INTERSECT query is wrapped in a derived table, the EXPLAIN FORMAT=JSON code walks the query tree via Query_expression::explain_query_term(). The INTERSECT query term produces a context type outside the expected {CTX_JOIN, CTX_UNION} set (likely a new context type for INTERSECT/EXCEPT), which violates the assertion. Call Stack subquery_ctx::set_child(context*) (opt_explain_json.cc:507) Explain_format_JSON::begin_context(...) (opt_explain_json.cc) Query_expression::explain_query_term(THD*, THD*, Query_term*) Query_expression::explain(THD*, THD const*) Explain::explain_subqueries() Explain::send() explain_query_specification(THD*, THD const*, Query_term*, ...) explain_query(THD*, THD const*, Query_expression*) Sql_cmd_dml::execute_inner(THD*) Sql_cmd_dml::execute(THD*) mysql_execute_command(THD*, bool) dispatch_sql_command(THD*, Parser_state*) How to repeat: SET SESSION debug_enable_extended_set_ops=ON; EXPLAIN FORMAT=JSON SELECT * FROM (SELECT 1 INTERSECT SELECT 2) AS t; Suggested fix: I was unable to reproduce this issue on version 8.0.32. Based on AI analysis, it appears to have been resolved by WL#349. I am submitting this report for official confirmation. mysql> SET SESSION debug_enable_extended_set_ops=ON; ERROR 1193 (HY000): Unknown system variable 'debug_enable_extended_set_ops' mysql> select version(); +--------------+ | version() | +--------------+ | 8.0.32-debug | +--------------+ 1 row in set (0.00 sec)