MDEV-24975 Server consumes extra 4G memory upon querying INFORMATION_SCHEMA.OPTIIMIZER_TRACE

if a query used no fields from an I_S table, we were creating a temp
table with one, first, field (as a table cannot have zero fields),
with its length truncated to 1.

Now - force also this dummy field to be a normal field, not a BLOB

mysql-test/main/opt_trace.result

@@ -8591,4 +8591,13 @@ JSON_DETAILED(JSON_EXTRACT(trace, '$**.range_scan_alternatives'))
 ]
 DROP TABLE t1;
 set optimizer_trace='enabled=off';
+#
+# MDEV-24975 Server consumes extra 4G memory upon querying INFORMATION_SCHEMA.OPTIIMIZER_TRACE
+#
+set max_session_mem_used=1024*1024*1024;
+select count(*) from information_schema.optimizer_trace;
+select * from information_schema.optimizer_trace;
+set max_session_mem_used=default;
+#
 # End of 10.4 tests
+#

mysql-test/main/opt_trace.test

@@ -623,4 +623,17 @@ SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.range_scan_alternatives')) from IN
 DROP TABLE t1;
 
 set optimizer_trace='enabled=off';
+
+--echo #
+--echo # MDEV-24975 Server consumes extra 4G memory upon querying INFORMATION_SCHEMA.OPTIIMIZER_TRACE
+--echo #
+set max_session_mem_used=1024*1024*1024;
+--disable_result_log
+select count(*) from information_schema.optimizer_trace;
+select * from information_schema.optimizer_trace;
+--enable_result_log
+set max_session_mem_used=default;
+
+--echo #
 --echo # End of 10.4 tests
+--echo #

sql/sql_show.cc

@@ -8767,6 +8767,7 @@ bool optimize_schema_tables_memory_usage(List<TABLE_LIST> &tables)
       {
         /* all fields were optimized away. Force a non-0-length row */
         table->s->reclength= to_recinfo->length= 1;
+        to_recinfo->type= FIELD_NORMAL;
         to_recinfo++;
       }
       p->recinfo= to_recinfo;