sql: refactor DISCARD so it runs during execution

Previously, it would run during planning, which is not correct, or at
least, makes it harder to reason about what's going on.

Release note: None

Release justification: low risk refactor

pkg/sql/discard.go

@@ -22,28 +22,39 @@ import (
 // Discard implements the DISCARD statement.
 // See https://www.postgresql.org/docs/9.6/static/sql-discard.html for details.
 func (p *planner) Discard(ctx context.Context, s *tree.Discard) (planNode, error) {
-	switch s.Mode {
+	return &discardNode{mode: s.Mode}, nil
+}
+
+type discardNode struct {
+	mode tree.DiscardMode
+}
+
+func (n *discardNode) Next(_ runParams) (bool, error) { return false, nil }
+func (n *discardNode) Values() tree.Datums            { return nil }
+func (n *discardNode) Close(_ context.Context)        {}
+func (n *discardNode) startExec(params runParams) error {
+	switch n.mode {
 	case tree.DiscardModeAll:
-		if !p.autoCommit {
-			return nil, pgerror.New(pgcode.ActiveSQLTransaction,
+		if !params.p.autoCommit {
+			return pgerror.New(pgcode.ActiveSQLTransaction,
 				"DISCARD ALL cannot run inside a transaction block")
 		}
 
 		// RESET ALL
-		if err := p.sessionDataMutatorIterator.applyOnEachMutatorError(
+		if err := params.p.sessionDataMutatorIterator.applyOnEachMutatorError(
 			func(m sessionDataMutator) error {
-				return resetSessionVars(ctx, m)
+				return resetSessionVars(params.ctx, m)
 			},
 		); err != nil {
-			return nil, err
+			return err
 		}
 
 		// DEALLOCATE ALL
-		p.preparedStatements.DeleteAll(ctx)
+		params.p.preparedStatements.DeleteAll(params.ctx)
 	default:
-		return nil, errors.AssertionFailedf("unknown mode for DISCARD: %d", s.Mode)
+		return errors.AssertionFailedf("unknown mode for DISCARD: %d", n.mode)
 	}
-	return newZeroNode(nil /* columns */), nil
+	return nil
 }
 
 func resetSessionVars(ctx context.Context, m sessionDataMutator) error {

pkg/sql/pgwire/testdata/pgtest/pgjdbc

@@ -42,13 +42,7 @@ send
 Query {"String": "DISCARD ALL"}
 ----
 
-until crdb_only ignore=ParameterStatus
-ReadyForQuery
-----
-{"Type":"CommandComplete","CommandTag":"DISCARD"}
-{"Type":"ReadyForQuery","TxStatus":"I"}
-
-until noncrdb_only ignore=ParameterStatus
+until ignore=ParameterStatus ignore=NoticeResponse
 ReadyForQuery
 ----
 {"Type":"CommandComplete","CommandTag":"DISCARD ALL"}

pkg/sql/sem/tree/stmt.go

@@ -853,7 +853,13 @@ func (*Discard) StatementReturnType() StatementReturnType { return Ack }
 func (*Discard) StatementType() StatementType { return TypeTCL }
 
 // StatementTag returns a short string identifying the type of statement.
-func (*Discard) StatementTag() string { return "DISCARD" }
+func (d *Discard) StatementTag() string {
+	switch d.Mode {
+	case DiscardModeAll:
+		return "DISCARD ALL"
+	}
+	return "DISCARD"
+}
 
 // StatementReturnType implements the Statement interface.
 func (n *DeclareCursor) StatementReturnType() StatementReturnType { return Ack }

pkg/sql/walk.go

@@ -393,6 +393,7 @@ var planNodeNames = map[reflect.Type]string{
 	reflect.TypeOf(&delayedNode{}):                             "virtual table",
 	reflect.TypeOf(&deleteNode{}):                              "delete",
 	reflect.TypeOf(&deleteRangeNode{}):                         "delete range",
+	reflect.TypeOf(&discardNode{}):                             "discard",
 	reflect.TypeOf(&distinctNode{}):                            "distinct",
 	reflect.TypeOf(&dropDatabaseNode{}):                        "drop database",
 	reflect.TypeOf(&dropExternalConnectionNode{}):              "drop external connection",