MDEV-30693: Assertion `dbl_records <= s->records' failed in apply_selectivity_for_table on SELECT

The crash happened due to rows=2 vs rows=1 difference between how the
estimate of number of rows in a derived table is computed in
TABLE_LIST::fetch_number_of_rows() and JOIN::add_keyuses_for_splitting().

Made JOIN::add_keyuses_for_splitting() use the result of computations in
TABLE_LIST::fetch_number_of_rows().

mysql-test/main/selectivity_innodb.result

@@ -2322,5 +2322,14 @@ b
 9
 DROP TABLE t;
 #
+# MDEV-30693: Assertion `dbl_records <= s->records' failed in apply_selectivity_for_table on SELECT
+#
+set @tmp_oucs= @@optimizer_use_condition_selectivity;
+CREATE TABLE t1 (c INT KEY) ENGINE=InnoDB;
+SELECT * FROM (SELECT * FROM t1) a JOIN (SELECT * FROM (SELECT * FROM t1 GROUP BY c) d WHERE c>1) b ON a.c=b.c;
+c	c
+DROP TABLE t1;
+SET optimizer_use_condition_selectivity=1;
+#
 # End of 11.0 tests
 #

mysql-test/main/selectivity_innodb.test

@@ -282,6 +282,15 @@ SELECT b FROM t WHERE a > 'a' GROUP BY b HAVING b >= 6 OR b <= 0;
 # Cleanup
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-30693: Assertion `dbl_records <= s->records' failed in apply_selectivity_for_table on SELECT
+--echo #
+set @tmp_oucs= @@optimizer_use_condition_selectivity;
+CREATE TABLE t1 (c INT KEY) ENGINE=InnoDB;
+SELECT * FROM (SELECT * FROM t1) a JOIN (SELECT * FROM (SELECT * FROM t1 GROUP BY c) d WHERE c>1) b ON a.c=b.c;
+DROP TABLE t1;
+SET optimizer_use_condition_selectivity=1;
+
 --echo #
 --echo # End of 11.0 tests
 --echo #

sql/opt_split.cc

@@ -741,7 +741,12 @@ void JOIN::add_keyuses_for_splitting()
   if (ext_keyuses_for_splitting->push(keyuse_ext_end))
     goto err;
 
-  spl_opt_info->unsplit_card= join_record_count;
+  /*
+    Use the number of rows that was computed by
+    TABLE_LIST::fetch_number_of_rows():
+  */
+  spl_opt_info->unsplit_card=
+    select_lex->master_unit()->derived->table->stat_records();
 
   rec_len= table->s->rec_buff_length;