MDEV-24690 Dropping primary key column from versioned table always fails with 1072

Exclude system-invisible key-parts from MDEV-11114 (04b288ae)
restriction.

mysql-test/suite/versioning/r/alter.result

@@ -693,3 +693,45 @@ delete from t1;
 set statement system_versioning_alter_history=keep for
 alter table t1 drop system versioning, modify column a tinyint;
 drop table t1;
+#
+# MDEV-24690 Dropping primary key column from versioned table always fails with 1072
+#
+create table t1 (a int, b int primary key) with system versioning;
+alter table t1 drop column b;
+create or replace table t1 (
+a int, b int primary key,
+row_start timestamp(6) as row start,
+row_end timestamp(6) as row end,
+period for system_time(row_start, row_end)
+) with system versioning;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) NOT NULL,
+  `row_start` timestamp(6) GENERATED ALWAYS AS ROW START,
+  `row_end` timestamp(6) GENERATED ALWAYS AS ROW END,
+  PRIMARY KEY (`b`,`row_end`),
+  PERIOD FOR SYSTEM_TIME (`row_start`, `row_end`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 WITH SYSTEM VERSIONING
+alter table t1 drop column b;
+ERROR 42000: Key column 'b' doesn't exist in table
+create or replace table t1 (
+a int, b int primary key,
+row_start timestamp(6) as row start invisible,
+row_end timestamp(6) as row end invisible,
+period for system_time(row_start, row_end)
+) with system versioning;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) NOT NULL,
+  `row_start` timestamp(6) GENERATED ALWAYS AS ROW START INVISIBLE,
+  `row_end` timestamp(6) GENERATED ALWAYS AS ROW END INVISIBLE,
+  PRIMARY KEY (`b`,`row_end`),
+  PERIOD FOR SYSTEM_TIME (`row_start`, `row_end`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 WITH SYSTEM VERSIONING
+alter table t1 drop column b;
+ERROR 42000: Key column 'b' doesn't exist in table
+drop table t1;

mysql-test/suite/versioning/t/alter.test

@@ -592,3 +592,32 @@ alter table t1 drop system versioning, modify column a tinyint;
 
 # cleanup
 drop table t1;
+
+--echo #
+--echo # MDEV-24690 Dropping primary key column from versioned table always fails with 1072
+--echo #
+create table t1 (a int, b int primary key) with system versioning;
+alter table t1 drop column b;
+
+create or replace table t1 (
+  a int, b int primary key,
+  row_start timestamp(6) as row start,
+  row_end timestamp(6) as row end,
+  period for system_time(row_start, row_end)
+) with system versioning;
+show create table t1;
+--error ER_KEY_COLUMN_DOES_NOT_EXITS
+alter table t1 drop column b;
+
+create or replace table t1 (
+a int, b int primary key,
+  row_start timestamp(6) as row start invisible,
+  row_end timestamp(6) as row end invisible,
+  period for system_time(row_start, row_end)
+) with system versioning;
+show create table t1;
+--error ER_KEY_COLUMN_DOES_NOT_EXITS
+alter table t1 drop column b;
+
+# cleanup
+drop table t1;

sql/sql_table.cc

@@ -8375,6 +8375,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
     }
 
     const char *dropped_key_part= NULL;
+    bool user_keyparts= false; // some user-defined keyparts left
     KEY_PART_INFO *key_part= key_info->key_part;
     key_parts.empty();
     bool delete_index_stat= FALSE;
@@ -8450,6 +8451,8 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
       key_parts.push_back(new (thd->mem_root) Key_part_spec(&cfield->field_name,
                                                             key_part_length, true),
                           thd->mem_root);
+      if (cfield->invisible < INVISIBLE_SYSTEM)
+        user_keyparts= true;
     }
     if (table->s->tmp_table == NO_TMP_TABLE)
     {
@@ -8493,7 +8496,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
           key_type= Key::PRIMARY;
         else
           key_type= Key::UNIQUE;
-        if (dropped_key_part)
+        if (dropped_key_part && user_keyparts)
         {
           my_error(ER_KEY_COLUMN_DOES_NOT_EXITS, MYF(0), dropped_key_part);
           goto err;