Online check constraint addition (#76)

* Online check constraint addition

README.md

@@ -43,10 +43,13 @@ $ pg-schema-diff plan --dsn "postgres://postgres:postgres@localhost:5432/postgre
 * The use of postgres native operations for zero-downtime migrations wherever possible:
   * Concurrent index builds
   * Online index replacement: If some index is changed, the new version will be built before the old version is dropped, preventing a window where no index is backing queries
+  * Online check constraint builds: Check constraints are added as `INVALID` before being validated, eliminating the need
+	for a long access-exclusive lock on the table
   * Prioritized index builds: Building new indexes is always prioritized over deleting old indexes
 * A comprehensive set of features to ensure the safety of planned migrations:
   * Operators warned of dangerous operations.
   * Migration plans are validated first against a temporary database exactly as they would be performed against the real database.
+* Strong support of partitions
 # Install
 ## CLI
 ```bash

internal/migration_acceptance_tests/check_constraint_cases_test.go

@@ -52,9 +52,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			);
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add check constraint with UDF dependency should error",
@@ -109,9 +106,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			);
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add multiple check constraints",
@@ -134,9 +128,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			);
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add check constraints to new column",
@@ -157,9 +148,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			);
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add check constraint with quoted identifiers",
@@ -182,9 +170,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			ALTER TABLE foobar ADD CONSTRAINT "BAR_CHECK" CHECK ( "Bar" < "ID" );
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add no inherit check constraint",
@@ -207,9 +192,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			ALTER TABLE foobar ADD CONSTRAINT bar_check CHECK ( bar > id ) NO INHERIT;
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Add No-Inherit, Not-Valid check constraint",
@@ -420,9 +402,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			ALTER TABLE foobar ADD CONSTRAINT bar_check CHECK ( bar > id );
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Alter an Inheritable check constraint to be no-inherit",
@@ -446,9 +425,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			ALTER TABLE foobar ADD CONSTRAINT bar_check CHECK ( bar > id ) NO INHERIT;
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Alter a check constraint expression",
@@ -470,9 +446,6 @@ var checkConstraintCases = []acceptanceTestCase{
 			);
 			`,
 		},
-		expectedHazardTypes: []diff.MigrationHazardType{
-			diff.MigrationHazardTypeAcquiresAccessExclusiveLock,
-		},
 	},
 	{
 		name: "Alter check constraint with UDF dependency should error",

pkg/diff/schema_migration_plan_test.go

@@ -838,6 +838,48 @@ var (
 			expectedStatements: nil,
 			expectedDiffErrIs:  errDuplicateIdentifier,
 		},
+		{
+			name: "Online check constraint build",
+			oldSchema: schema.Schema{
+				Tables: []schema.Table{
+					{
+						Name: "foobar",
+						Columns: []schema.Column{
+							{Name: "id", Type: "integer"},
+						},
+						ReplicaIdentity: schema.ReplicaIdentityDefault,
+					},
+				},
+			},
+			newSchema: schema.Schema{
+				Tables: []schema.Table{
+					{
+						Name: "foobar",
+						Columns: []schema.Column{
+							{Name: "id", Type: "integer"},
+						},
+						CheckConstraints: []schema.CheckConstraint{
+							{Name: "id_check", Expression: "(id > 0)", IsInheritable: true, IsValid: true},
+						},
+						ReplicaIdentity: schema.ReplicaIdentityDefault,
+					},
+				},
+			},
+			expectedStatements: []Statement{
+				{
+					DDL:         "ALTER TABLE \"public\".\"foobar\" ADD CONSTRAINT \"id_check\" CHECK((id > 0)) NOT VALID",
+					Timeout:     statementTimeoutDefault,
+					LockTimeout: lockTimeoutDefault,
+					Hazards:     nil,
+				},
+				{
+					DDL:         "ALTER TABLE \"public\".\"foobar\" VALIDATE CONSTRAINT \"id_check\"",
+					Timeout:     statementTimeoutDefault,
+					LockTimeout: lockTimeoutDefault,
+					Hazards:     nil,
+				},
+			},
+		},
 		{
 			name: "Invalid check constraint made valid",
 			oldSchema: schema.Schema{

pkg/diff/sql_generator.go

@@ -1444,14 +1444,29 @@ type checkConstraintSQLGenerator struct {
 }
 
 func (csg *checkConstraintSQLGenerator) Add(con schema.CheckConstraint) ([]Statement, error) {
-	var hazards []MigrationHazard
-
 	// UDF's in check constraints are a bad idea. Check constraints are not re-validated
 	// if the UDF changes, so it's not really a safe practice. We won't support it for now
 	if len(con.DependsOnFunctions) > 0 {
 		return nil, fmt.Errorf("check constraints that depend on UDFs: %w", ErrNotImplemented)
 	}
 
+	var stmts []Statement
+	if !con.IsValid || csg.isNewTable {
+		stmts = append(stmts, csg.createCheckConstraintStatement(con))
+	} else {
+		// If the check constraint is not on a new table and is marked as valid, we should:
+		// 1. Build the constraint as invalid
+		// 2. Validate the constraint
+		con.IsValid = false
+		stmts = append(stmts, csg.createCheckConstraintStatement(con))
+		stmts = append(stmts, validateConstraintStatement(csg.tableName, schema.EscapeIdentifier(con.Name)))
+	}
+
+	return stmts, nil
+}
+
+func (csg *checkConstraintSQLGenerator) createCheckConstraintStatement(con schema.CheckConstraint) Statement {
+	var hazards []MigrationHazard
 	sb := strings.Builder{}
 	sb.WriteString(fmt.Sprintf("%s CHECK(%s)",
 		addConstraintPrefix(csg.tableName, schema.EscapeIdentifier(con.Name)), con.Expression))
@@ -1461,20 +1476,20 @@ func (csg *checkConstraintSQLGenerator) Add(con schema.CheckConstraint) ([]State
 
 	if !con.IsValid {
 		sb.WriteString(" NOT VALID")
-	} else if !csg.isNewTable {
+	} else {
 		hazards = append(hazards, MigrationHazard{
 			Type: MigrationHazardTypeAcquiresAccessExclusiveLock,
 			Message: "This will lock reads and writes to the owning table while the constraint is being added. " +
 				"Instead, consider adding the constraint as NOT VALID and validating it later.",
 		})
 	}
 
-	return []Statement{{
+	return Statement{
 		DDL:         sb.String(),
 		Timeout:     statementTimeoutDefault,
 		LockTimeout: lockTimeoutDefault,
 		Hazards:     hazards,
-	}}, nil
+	}
 }
 
 func (csg *checkConstraintSQLGenerator) Delete(con schema.CheckConstraint) ([]Statement, error) {