MDEV-18035 Failing assertion on DELETE

instant_alter_column_possible(): Do not allow instant removal of NOT NULL
attribute from a column that belongs to the key of the clustered index.

mysql-test/suite/innodb/r/instant_alter.result

@@ -615,6 +615,11 @@ DROP TABLE t1;
 CREATE TABLE t1 (f1 VARCHAR(1), f2 VARCHAR(2)) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
 ALTER TABLE t1 MODIFY f2 VARCHAR (8) FIRST;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT UNIQUE, b INT UNIQUE, PRIMARY KEY(a,b)) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+ALTER TABLE t1 DROP PRIMARY KEY;
+ALTER TABLE t1 CHANGE COLUMN a a INT;
+DELETE FROM t1 WHERE a = NULL OR a IS NULL;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1176,6 +1181,11 @@ DROP TABLE t1;
 CREATE TABLE t1 (f1 VARCHAR(1), f2 VARCHAR(2)) ENGINE=InnoDB ROW_FORMAT=COMPACT;
 ALTER TABLE t1 MODIFY f2 VARCHAR (8) FIRST;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT UNIQUE, b INT UNIQUE, PRIMARY KEY(a,b)) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+ALTER TABLE t1 DROP PRIMARY KEY;
+ALTER TABLE t1 CHANGE COLUMN a a INT;
+DELETE FROM t1 WHERE a = NULL OR a IS NULL;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1737,6 +1747,11 @@ DROP TABLE t1;
 CREATE TABLE t1 (f1 VARCHAR(1), f2 VARCHAR(2)) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
 ALTER TABLE t1 MODIFY f2 VARCHAR (8) FIRST;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT UNIQUE, b INT UNIQUE, PRIMARY KEY(a,b)) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+ALTER TABLE t1 DROP PRIMARY KEY;
+ALTER TABLE t1 CHANGE COLUMN a a INT;
+DELETE FROM t1 WHERE a = NULL OR a IS NULL;
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status

mysql-test/suite/innodb/t/instant_alter.test

@@ -490,6 +490,13 @@ eval CREATE TABLE t1 (f1 VARCHAR(1), f2 VARCHAR(2)) $engine;
 ALTER TABLE t1 MODIFY f2 VARCHAR (8) FIRST;
 DROP TABLE t1;
 
+# MDEV-18035 Failing assertion on DELETE
+eval CREATE TABLE t1 (a INT UNIQUE, b INT UNIQUE, PRIMARY KEY(a,b)) $engine;
+ALTER TABLE t1 DROP PRIMARY KEY;
+ALTER TABLE t1 CHANGE COLUMN a a INT;
+DELETE FROM t1 WHERE a = NULL OR a IS NULL;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -1372,13 +1372,15 @@ check_v_col_in_order(
 /** Determine if an instant operation is possible for altering columns.
 @param[in]	ib_table	InnoDB table definition
 @param[in]	ha_alter_info	the ALTER TABLE operation
-@param[in]	table		table definition before ALTER TABLE */
+@param[in]	table		table definition before ALTER TABLE
+@param[in]	table		table definition after ALTER TABLE */
 static
 bool
 instant_alter_column_possible(
 	const dict_table_t&		ib_table,
 	const Alter_inplace_info*	ha_alter_info,
-	const TABLE*			table)
+	const TABLE*			table,
+	const TABLE*			altered_table)
 {
 	if (!ib_table.supports_instant()) {
 		return false;
@@ -1470,10 +1472,36 @@ instant_alter_column_possible(
 		return false;
 	}
 
-	if ((ha_alter_info->handler_flags
-	     & ALTER_COLUMN_NULLABLE)
-	    && ib_table.not_redundant()) {
-		return false;
+	if (ha_alter_info->handler_flags & ALTER_COLUMN_NULLABLE) {
+		if (ib_table.not_redundant()) {
+			return false;
+		}
+
+		const dict_index_t* pk = ib_table.indexes.start;
+		Field** af = altered_table->field;
+		Field** const end = altered_table->field
+			+ altered_table->s->fields;
+		for (unsigned c = 0; af < end; af++) {
+			if (!(*af)->stored_in_db()) {
+				continue;
+			}
+
+			const dict_col_t* col = dict_table_get_nth_col(
+				&ib_table, c++);
+
+			if (!col->ord_part || col->is_nullable()
+			    || !(*af)->real_maybe_null()) {
+				continue;
+			}
+
+			/* The column would be changed from NOT NULL.
+			Ensure that it is not a clustered index key. */
+			for (auto i = pk->n_uniq; i--; ) {
+				if (pk->fields[i].col == col) {
+					return false;
+				}
+			}
+		}
 	}
 
 	return true;
@@ -1784,7 +1812,7 @@ ha_innobase::check_if_supported_inplace_alter(
 	}
 
 	const bool supports_instant = instant_alter_column_possible(
-		*m_prebuilt->table, ha_alter_info, table);
+		*m_prebuilt->table, ha_alter_info, table, altered_table);
 	bool	add_drop_v_cols = false;
 
 	/* If there is add or drop virtual columns, we will support operations
@@ -6285,7 +6313,7 @@ prepare_inplace_alter_table_dict(
 		    || !ctx->new_table->persistent_autoinc);
 
 	if (ctx->need_rebuild() && instant_alter_column_possible(
-		    *user_table, ha_alter_info, old_table)
+		    *user_table, ha_alter_info, old_table, altered_table)
 #if 1 // MDEV-17459: adjust fts_fetch_doc_from_rec() and friends; remove this
 		&& !innobase_fulltext_exist(altered_table)
 #endif