Fix ALTER TABLE ... DROP CONSTRAINT for UNIQUE constraints

server/analyzer/convert_drop_unique_constraint.go

@@ -0,0 +1,59 @@
+// Copyright 2025 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package analyzer
+
+import (
+	"github.com/dolthub/go-mysql-server/sql"
+	"github.com/dolthub/go-mysql-server/sql/analyzer"
+	"github.com/dolthub/go-mysql-server/sql/plan"
+	"github.com/dolthub/go-mysql-server/sql/transform"
+)
+
+// convertDropUniqueConstraint converts a DropConstraint node dropping a unique constraint into
+// an AlterIndex node with IndexAction_Drop that GMS can process to remove the unique index.
+func convertDropUniqueConstraint(ctx *sql.Context, _ *analyzer.Analyzer, n sql.Node, _ *plan.Scope, _ analyzer.RuleSelector, _ *sql.QueryFlags) (sql.Node, transform.TreeIdentity, error) {
+	return transform.Node(n, func(n sql.Node) (sql.Node, transform.TreeIdentity, error) {
+		dropConstraint, ok := n.(*plan.DropConstraint)
+		if !ok {
+			return n, transform.SameTree, nil
+		}
+
+		rt, ok := dropConstraint.Child.(*plan.ResolvedTable)
+		if !ok {
+			return nil, transform.SameTree, analyzer.ErrInAnalysis.New(
+				"Expected a TableNode for ALTER TABLE DROP CONSTRAINT statement")
+		}
+
+		table := rt.Table
+		if it, ok := table.(sql.IndexAddressableTable); ok {
+			indexes, err := it.GetIndexes(ctx)
+			if err != nil {
+				return nil, transform.SameTree, err
+			}
+			for _, index := range indexes {
+				if index.IsUnique() && index.ID() != "PRIMARY" && index.ID() == dropConstraint.Name {
+					alterDropIndex := plan.NewAlterDropIndex(rt.Database(), rt, dropConstraint.IfExists, dropConstraint.Name)
+					newNode, err := alterDropIndex.WithTargetSchema(rt.Schema())
+					if err != nil {
+						return n, transform.SameTree, err
+					}
+					return newNode, transform.NewTree, nil
+				}
+			}
+		}
+
+		return n, transform.SameTree, nil
+	})
+}

server/analyzer/init.go

@@ -36,6 +36,7 @@ const (
 	ruleId_AssignTriggers                                                // assignTriggers
 	ruleId_AssignUpdateCasts                                             // assignUpdateCasts
 	ruleId_ConvertDropPrimaryKeyConstraint                               // convertDropPrimaryKeyConstraint
+	ruleId_ConvertDropUniqueConstraint                                   // convertDropUniqueConstraint
 	ruleId_GenerateForeignKeyName                                        // generateForeignKeyName
 	ruleId_ReplaceIndexedTables                                          // replaceIndexedTables
 	ruleId_ReplaceNode                                                   // replaceNode
@@ -68,7 +69,8 @@ func Init() {
 
 	analyzer.OnceBeforeDefault = append([]analyzer.Rule{
 		{Id: ruleId_ApplyTablesForAnalyzeAllTables, Apply: applyTablesForAnalyzeAllTables},
-		{Id: ruleId_ConvertDropPrimaryKeyConstraint, Apply: convertDropPrimaryKeyConstraint}},
+		{Id: ruleId_ConvertDropPrimaryKeyConstraint, Apply: convertDropPrimaryKeyConstraint},
+		{Id: ruleId_ConvertDropUniqueConstraint, Apply: convertDropUniqueConstraint}},
 		analyzer.OnceBeforeDefault...)
 
 	// We remove several validation rules and substitute our own

testing/go/alter_table_test.go

@@ -189,6 +189,120 @@ func TestAlterTable(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name: "Drop unique constraint added via ALTER TABLE",
+			SetUpScript: []string{
+				"CREATE TABLE t1 (pk int PRIMARY KEY, c1 int, c2 int);",
+				"INSERT INTO t1 VALUES (1, 10, 20);",
+				"ALTER TABLE t1 ADD CONSTRAINT uniq_c1c2 UNIQUE (c1, c2);",
+			},
+			Assertions: []ScriptTestAssertion{
+				{
+					Query:       "INSERT INTO t1 VALUES (2, 10, 20);",
+					ExpectedErr: "unique",
+				},
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT uniq_c1c2;",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "INSERT INTO t1 VALUES (2, 10, 20);",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "SELECT * FROM t1 ORDER BY pk;",
+					Expected: []sql.Row{{1, 10, 20}, {2, 10, 20}},
+				},
+			},
+		},
+		{
+			Name: "Drop unique constraint defined inline in CREATE TABLE",
+			SetUpScript: []string{
+				"CREATE TABLE t1 (pk int PRIMARY KEY, c1 int, c2 int, CONSTRAINT uniq_inline UNIQUE (c1, c2));",
+				"INSERT INTO t1 VALUES (1, 10, 20);",
+			},
+			Assertions: []ScriptTestAssertion{
+				{
+					Query:       "INSERT INTO t1 VALUES (2, 10, 20);",
+					ExpectedErr: "unique",
+				},
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT uniq_inline;",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "INSERT INTO t1 VALUES (2, 10, 20);",
+					Expected: []sql.Row{},
+				},
+			},
+		},
+		{
+			Name: "Drop unique constraint with IF EXISTS",
+			SetUpScript: []string{
+				"CREATE TABLE t1 (pk int PRIMARY KEY, c1 int);",
+				"ALTER TABLE t1 ADD CONSTRAINT uniq_c1 UNIQUE (c1);",
+			},
+			Assertions: []ScriptTestAssertion{
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT uniq_c1;",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:       "ALTER TABLE t1 DROP CONSTRAINT uniq_c1;",
+					ExpectedErr: "does not exist",
+				},
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT IF EXISTS uniq_c1;",
+					Expected: []sql.Row{},
+				},
+			},
+		},
+		{
+			Name: "Drop unique constraint on single column",
+			SetUpScript: []string{
+				"CREATE TABLE t1 (pk int PRIMARY KEY, email varchar(256), CONSTRAINT uniq_email UNIQUE (email));",
+				"INSERT INTO t1 VALUES (1, 'a@b.com');",
+			},
+			Assertions: []ScriptTestAssertion{
+				{
+					Query:       "INSERT INTO t1 VALUES (2, 'a@b.com');",
+					ExpectedErr: "unique",
+				},
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT uniq_email;",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "INSERT INTO t1 VALUES (2, 'a@b.com');",
+					Expected: []sql.Row{},
+				},
+			},
+		},
+		{
+			Name: "Drop and re-add unique constraint with different columns",
+			SetUpScript: []string{
+				"CREATE TABLE t1 (pk int PRIMARY KEY, c1 int, c2 int, c3 int);",
+				"ALTER TABLE t1 ADD CONSTRAINT uniq1 UNIQUE (c1, c2);",
+			},
+			Assertions: []ScriptTestAssertion{
+				{
+					Query:    "ALTER TABLE t1 DROP CONSTRAINT uniq1;",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "ALTER TABLE t1 ADD CONSTRAINT uniq1 UNIQUE (c1, c2, c3);",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:    "INSERT INTO t1 VALUES (1, 10, 20, 30);",
+					Expected: []sql.Row{},
+				},
+				{
+					Query:       "INSERT INTO t1 VALUES (2, 10, 20, 30);",
+					ExpectedErr: "unique",
+				},
+			},
+		},
 		{
 			Name: "Add Primary Key",
 			SetUpScript: []string{