Use expression deparsing to convert column DEFAULT expressions (#535)

Deparse column `DEFAULT` expressions using `pg_query_go.DeparseExpr` to
allow conversion of arbitrary expressions in column `DEFAULT`s to
`pgroll` migrations.

Replace manual deparsing of literal nodes with use of `DeparseExpr` to
allow deparsing of any column `DEFAULT` expression.

Update test cases:
* `ALTER TABLE foo ALTER COLUMN bar SET DEFAULT now()` is now
representable as an `OpAlterColumn`; no need to fall back to raw SQL
anymore.
* Add a new testcase: `ALTER TABLE foo ALTER COLUMN bar SET DEFAULT
(first_name || ' ' || last_name)` to demonstrate conversion of a column
`DEFAULT` using a more complex expression.

Part of #504

pkg/sql2pgroll/alter_table.go

@@ -4,7 +4,6 @@ package sql2pgroll
 
 import (
 	"fmt"
-	"strconv"
 
 	"github.com/oapi-codegen/nullable"
 	pgq "github.com/xataio/pg_query_go/v6"
@@ -272,29 +271,16 @@ func convertAlterTableSetColumnDefault(stmt *pgq.AlterTableStmt, cmd *pgq.AlterT
 			operation.Default = nullable.NewNullNullable[string]()
 			return operation, nil
 		}
-
-		// We have a constant
-		switch v := c.GetVal().(type) {
-		case *pgq.A_Const_Sval:
-			operation.Default = nullable.NewNullableWithValue(fmt.Sprintf("'%s'", v.Sval.GetSval()))
-		case *pgq.A_Const_Ival:
-			operation.Default = nullable.NewNullableWithValue(strconv.FormatInt(int64(v.Ival.Ival), 10))
-		case *pgq.A_Const_Fval:
-			operation.Default = nullable.NewNullableWithValue(v.Fval.Fval)
-		case *pgq.A_Const_Boolval:
-			operation.Default = nullable.NewNullableWithValue(strconv.FormatBool(v.Boolval.Boolval))
-		case *pgq.A_Const_Bsval:
-			operation.Default = nullable.NewNullableWithValue(fmt.Sprintf("'%s'", v.Bsval.GetBsval()))
-		default:
-			return nil, fmt.Errorf("unknown constant type: %T", c.GetVal())
-		}
-
-		return operation, nil
 	}
 
+	// We're setting it to an expression
 	if cmd.GetDef() != nil {
-		// We're setting it to something other than a constant
-		return nil, nil
+		def, err := pgq.DeparseExpr(cmd.GetDef())
+		if err != nil {
+			return nil, fmt.Errorf("failed to deparse expression: %w", err)
+		}
+		operation.Default = nullable.NewNullableWithValue(def)
+		return operation, nil
 	}
 
 	// We're not setting it to anything, which is the case when we are dropping it

pkg/sql2pgroll/alter_table_test.go

@@ -64,6 +64,14 @@ func TestConvertAlterTableStatements(t *testing.T) {
 			sql:        "ALTER TABLE foo ALTER COLUMN bar SET DEFAULT null",
 			expectedOp: expect.AlterColumnOp7,
 		},
+		{
+			sql:        "ALTER TABLE foo ALTER COLUMN bar SET DEFAULT now()",
+			expectedOp: expect.AlterColumnOp11,
+		},
+		{
+			sql:        "ALTER TABLE foo ALTER COLUMN bar SET DEFAULT (first_name || ' ' || last_name)",
+			expectedOp: expect.AlterColumnOp12,
+		},
 		{
 			sql:        "ALTER TABLE foo ADD CONSTRAINT bar UNIQUE (a)",
 			expectedOp: expect.CreateConstraintOp1,
@@ -146,9 +154,6 @@ func TestUnconvertableAlterTableStatements(t *testing.T) {
 		"ALTER TABLE foo DROP COLUMN bar CASCADE",
 		"ALTER TABLE foo DROP COLUMN IF EXISTS bar",
 
-		// Non literal default values
-		"ALTER TABLE foo ALTER COLUMN bar SET DEFAULT now()",
-
 		// Unsupported foreign key statements
 		"ALTER TABLE foo ADD CONSTRAINT fk_bar_cd FOREIGN KEY (a, b) REFERENCES bar (c, d) ON UPDATE RESTRICT;",
 		"ALTER TABLE foo ADD CONSTRAINT fk_bar_cd FOREIGN KEY (a, b) REFERENCES bar (c, d) ON UPDATE CASCADE;",

pkg/sql2pgroll/expect/alter_column.go

@@ -82,7 +82,23 @@ var AlterColumnOp9 = &migrations.OpAlterColumn{
 var AlterColumnOp10 = &migrations.OpAlterColumn{
 	Table:   "foo",
 	Column:  "bar",
-	Default: nullable.NewNullableWithValue("'b0101'"),
+	Default: nullable.NewNullableWithValue("b'0101'"),
+	Up:      sql2pgroll.PlaceHolderSQL,
+	Down:    sql2pgroll.PlaceHolderSQL,
+}
+
+var AlterColumnOp11 = &migrations.OpAlterColumn{
+	Table:   "foo",
+	Column:  "bar",
+	Default: nullable.NewNullableWithValue("now()"),
+	Up:      sql2pgroll.PlaceHolderSQL,
+	Down:    sql2pgroll.PlaceHolderSQL,
+}
+
+var AlterColumnOp12 = &migrations.OpAlterColumn{
+	Table:   "foo",
+	Column:  "bar",
+	Default: nullable.NewNullableWithValue("(first_name || ' ') || last_name"),
 	Up:      sql2pgroll.PlaceHolderSQL,
 	Down:    sql2pgroll.PlaceHolderSQL,
 }