MDEV-25353 JSON_TABLE: Illegal mix of collations upon executing PS once, or SP/function twice.

Do not adapt to the argument charset anymore. Just use the utf8mb4 as
the default cherset.

mysql-test/suite/json/r/json_table_mysql.result

@@ -1081,7 +1081,7 @@ SELECT col_varchar_key FROM JSON_TABLE(
 (col_varchar_key VARCHAR(10) PATH "$.col_key")) AS innr1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
-2	MATERIALIZED	innr1	ALL	NULL	NULL	NULL	NULL	40	Table function: json_table
+2	DEPENDENT SUBQUERY	innr1	ALL	NULL	NULL	NULL	NULL	40	Table function: json_table; Using where
 SELECT * FROM t1 WHERE col_varchar_key IN (
 SELECT col_varchar_key FROM JSON_TABLE(
 '[{"col_key": 1},{"col_key": 2}]', "$[*]" COLUMNS
@@ -1094,8 +1094,7 @@ SELECT col_varchar_key FROM JSON_TABLE(
 (col_varchar_key VARCHAR(10) PATH "$.col_key")) AS innr1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
-1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	13	func	1	Using where
-2	MATERIALIZED	innr1	ALL	NULL	NULL	NULL	NULL	40	Table function: json_table
+1	PRIMARY	innr1	ALL	NULL	NULL	NULL	NULL	40	Table function: json_table; Using where; FirstMatch(t1); Using join buffer (flat, BNL join)
 DROP TABLE t1;
 #
 # Bug#26711551: WL8867:CONDITIONAL JUMP IN JSON_TABLE_COLUMN::CLEANUP
@@ -1422,7 +1421,7 @@ FROM JSON_TABLE('"test"', '$' COLUMNS(col VARCHAR(10) PATH '$')) AS a;
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `col` varchar(10) CHARACTER SET ascii DEFAULT NULL
+  `col` varchar(10) CHARACTER SET utf8mb4 DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 SET @@SESSION.collation_connection = latin1_bin;
 CREATE TABLE t2 SELECT a.col
@@ -1430,7 +1429,7 @@ FROM JSON_TABLE('"test"', '$' COLUMNS(col VARCHAR(10) PATH '$')) AS a;
 SHOW CREATE TABLE t2;
 Table	Create Table
 t2	CREATE TABLE `t2` (
-  `col` varchar(10) CHARACTER SET latin1 COLLATE latin1_bin DEFAULT NULL
+  `col` varchar(10) CHARACTER SET utf8mb4 DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1, t2;
 SET @@SESSION.character_set_connection = DEFAULT;

mysql-test/suite/json/t/json_table.test

@@ -735,6 +735,18 @@ SELECT * FROM JSON_TABLE('{}', '$' COLUMNS(
       a TEXT EXISTS PATH '$', b VARCHAR(40) PATH '$', c BIT(60) PATH '$', d VARCHAR(60) PATH '$', e BIT(62) PATH '$',
       f FOR ORDINALITY, g INT PATH '$', h VARCHAR(36) PATH '$', i DATE PATH '$', j CHAR(4) PATH '$'
       )) AS jt;
+
+--echo #
+--echo # MDEV-25373 JSON_TABLE: Illegal mix of collations upon executing PS once, or SP/function twice.
+--echo #
+SELECT * FROM JSON_TABLE (CONVERT('[1,2]' USING koi8u), '$[*]' COLUMNS(a CHAR(8) PATH '$')) AS jt1 NATURAL JOIN JSON_TABLE (CONVERT('[2,3]' USING eucjpms), '$[*]' COLUMNS(a CHAR(8) PATH '$')) AS jt2;
+ 
+PREPARE stmt1 FROM "
+SELECT * FROM JSON_TABLE (CONVERT('[1,2]' USING koi8u), '$[*]' COLUMNS(a CHAR(8) PATH '$')) AS jt1 NATURAL JOIN JSON_TABLE (CONVERT('[2,3]' USING eucjpms), '$[*]' COLUMNS(a CHAR(8) PATH '$')) AS jt2;
+";
+EXECUTE stmt1;
+DEALLOCATE PREPARE stmt1;
+
 --echo #
 --echo # End of 10.6 tests
 --echo #

sql/json_table.cc

@@ -1077,40 +1077,6 @@ int Table_function_json_table::setup(THD *thd, TABLE_LIST *sql_table,
       return TRUE;
   }
 
-  {
-    /*
-      The m_json defines it's charset during the fix_fields stage so we're
-      changing the field's current charsets with it.
-      The complicated part is that the length of the field can be changed so
-      in this case we should move all the consequent fiedlds.
-    */
-    List_iterator_fast<Json_table_column> jc_i(m_columns);
-    int field_offset= 0;
-    Field *f;
-    for (uint i= 0; (f= t->field[i]); i++)
-    {
-      Json_table_column *jc= jc_i++;
-      uint32 old_pack_length= f->pack_length();
-
-      f->change_charset(
-                jc->m_explicit_cs ? jc->m_explicit_cs : m_json->collation);
-
-      if (field_offset)
-        f->move_field(f->ptr + field_offset, f->null_ptr, f->null_bit);
-
-      f->reset();
-
-      field_offset= (field_offset + f->pack_length()) - old_pack_length;
-
-      /*
-        The m_field->charset is going to be reused if it's the prepared
-        statement running several times. So should restored the original
-        value.
-      */
-      jc->m_field->charset= jc->m_explicit_cs;
-    }
-    store_record(t, s->default_values);
-  }
   return FALSE;
 }