sql/mysql: fix the way we ignore indexes that were created automatica…

…lly by foreign-keys

sql/mysql/diff.go

@@ -96,20 +96,19 @@ func (*diff) ReferenceChanged(from, to schema.ReferenceOption) bool {
 
 // Normalize implements the sqlx.Normalizer interface.
 func (d *diff) Normalize(from, to *schema.Table) {
-	for i, idx := range from.Indexes {
-		if _, ok := to.Index(idx.Name); ok {
-			continue
-		}
+	indexes := make([]*schema.Index, 0, len(from.Indexes))
+	for _, idx := range from.Indexes {
 		// MySQL requires that foreign key columns be indexed; Therefore, if the child
 		// table is defined on non-indexed columns, an index is automatically created
 		// to satisfy the constraint.
 		// Therefore, if no such key was defined on the desired state, the diff will
 		// recommend to drop it on migration. Therefore, we fix it by dropping it from
 		// the current state manually.
-		if keySupportsFK(from, idx) {
-			from.Indexes = append(from.Indexes[:i], from.Indexes[i+1:]...)
+		if _, ok := to.Index(idx.Name); ok || !keySupportsFK(from, idx) {
+			indexes = append(indexes, idx)
 		}
 	}
+	from.Indexes = indexes
 }
 
 // collationChange returns the schema change for migrating the collation if

sql/mysql/migrate.go

@@ -535,28 +535,31 @@ func Build(phrase string) *sqlx.Builder {
 // skipAutoChanges filters unnecessary changes that are automatically
 // happened by the database when ALTER TABLE is executed.
 func skipAutoChanges(changes []schema.Change) []schema.Change {
-	dropC := make(map[string]bool)
+	var (
+		dropC   = make(map[string]bool)
+		planned = make([]schema.Change, 0, len(changes))
+	)
 	for _, c := range changes {
 		if c, ok := c.(*schema.DropColumn); ok {
 			dropC[c.C.Name] = true
 		}
 	}
-search:
 	for i, c := range changes {
 		// Simple case for skipping key dropping, if its columns are dropped.
 		// https://dev.mysql.com/doc/refman/8.0/en/alter-table.html#alter-table-add-drop-column
 		c, ok := c.(*schema.DropIndex)
 		if !ok {
+			planned = append(planned, changes[i])
 			continue
 		}
 		for _, p := range c.I.Parts {
 			if p.C == nil || !dropC[p.C.Name] {
-				continue search
+				planned = append(planned, c)
+				break
 			}
 		}
-		changes = append(changes[:i], changes[i+1:]...)
 	}
-	return changes
+	return planned
 }
 
 // checks writes the CHECK constraint to the builder.

sql/postgres/migrate.go

@@ -622,36 +622,39 @@ func Build(phrase string) *sqlx.Builder {
 // skipAutoChanges filters unnecessary changes that are automatically
 // happened by the database when ALTER TABLE is executed.
 func skipAutoChanges(changes []schema.Change) []schema.Change {
-	dropC := make(map[string]bool)
+	var (
+		dropC   = make(map[string]bool)
+		planned = make([]schema.Change, 0, len(changes))
+	)
 	for _, c := range changes {
 		if c, ok := c.(*schema.DropColumn); ok {
 			dropC[c.C.Name] = true
 		}
 	}
-	for i, c := range changes {
+search:
+	for _, c := range changes {
 		switch c := c.(type) {
 		// Indexes involving the column are automatically dropped
 		// with it. This true for multi-columns indexes as well.
 		// See https://www.postgresql.org/docs/current/sql-altertable.html
 		case *schema.DropIndex:
 			for _, p := range c.I.Parts {
-				if p.C == nil && dropC[p.C.Name] {
-					changes = append(changes[:i], changes[i+1:]...)
-					break
+				if p.C != nil && dropC[p.C.Name] {
+					continue search
 				}
 			}
 		// Simple case for skipping constraint dropping,
 		// if the child table columns were dropped.
 		case *schema.DropForeignKey:
 			for _, c := range c.F.Columns {
 				if dropC[c.Name] {
-					changes = append(changes[:i], changes[i+1:]...)
-					break
+					continue search
 				}
 			}
 		}
+		planned = append(planned, c)
 	}
-	return changes
+	return planned
 }
 
 // checks writes the CHECK constraint to the builder.