sql: add SHOW COMMIT TIMESTAMP to retrieve a causality token

docs/generated/sql/bnf/BUILD.bazel

@@ -210,6 +210,7 @@ FILES = [
     "show_backup",
     "show_cluster_setting",
     "show_columns_stmt",
+    "show_commit_timestamp_stmt",
     "show_constraints_stmt",
     "show_create_stmt",
     "show_create_schedules_stmt",

docs/generated/sql/bnf/show_commit_timestamp_stmt.bnf

@@ -0,0 +1,2 @@
+show_commit_timestamp_stmt ::=
+	'SHOW' 'COMMIT' 'TIMESTAMP'

docs/generated/sql/bnf/stmt_block.bnf

@@ -30,6 +30,7 @@ stmt_without_legacy_transaction ::=
 	| fetch_cursor_stmt
 	| move_cursor_stmt
 	| unlisten_stmt
+	| show_commit_timestamp_stmt
 
 legacy_transaction_stmt ::=
 	legacy_begin_stmt
@@ -160,6 +161,9 @@ unlisten_stmt ::=
 	'UNLISTEN' type_name
 	| 'UNLISTEN' '*'
 
+show_commit_timestamp_stmt ::=
+	'SHOW' 'COMMIT' 'TIMESTAMP'
+
 legacy_begin_stmt ::=
 	'BEGIN' opt_transaction begin_transaction
 

docs/generated/sql/bnf/stmt_without_legacy_transaction.bnf

@@ -21,3 +21,4 @@ stmt_without_legacy_transaction ::=
 	| fetch_cursor_stmt
 	| move_cursor_stmt
 	| unlisten_stmt
+	| show_commit_timestamp_stmt

pkg/gen/bnf.bzl

@@ -210,6 +210,7 @@ BNF_SRCS = [
   "//docs/generated/sql/bnf:show_backup.bnf",
   "//docs/generated/sql/bnf:show_cluster_setting.bnf",
   "//docs/generated/sql/bnf:show_columns_stmt.bnf",
+  "//docs/generated/sql/bnf:show_commit_timestamp_stmt.bnf",
   "//docs/generated/sql/bnf:show_constraints_stmt.bnf",
   "//docs/generated/sql/bnf:show_create_external_connections_stmt.bnf",
   "//docs/generated/sql/bnf:show_create_schedules_stmt.bnf",

pkg/gen/diagrams.bzl

@@ -208,6 +208,7 @@ DIAGRAMS_SRCS = [
   "//docs/generated/sql/bnf:show_backup.html",
   "//docs/generated/sql/bnf:show_cluster_setting.html",
   "//docs/generated/sql/bnf:show_columns.html",
+  "//docs/generated/sql/bnf:show_commit_timestamp.html",
   "//docs/generated/sql/bnf:show_constraints.html",
   "//docs/generated/sql/bnf:show_create.html",
   "//docs/generated/sql/bnf:show_create_external_connections.html",

pkg/gen/docs.bzl

@@ -222,6 +222,7 @@ DOCS_SRCS = [
   "//docs/generated/sql/bnf:show_backup.bnf",
   "//docs/generated/sql/bnf:show_cluster_setting.bnf",
   "//docs/generated/sql/bnf:show_columns_stmt.bnf",
+  "//docs/generated/sql/bnf:show_commit_timestamp_stmt.bnf",
   "//docs/generated/sql/bnf:show_constraints_stmt.bnf",
   "//docs/generated/sql/bnf:show_create_external_connections_stmt.bnf",
   "//docs/generated/sql/bnf:show_create_schedules_stmt.bnf",

pkg/sql/BUILD.bazel

@@ -44,6 +44,7 @@ go_library(
         "conn_executor_exec.go",
         "conn_executor_prepare.go",
         "conn_executor_savepoints.go",
+        "conn_executor_show_commit_timestamp.go",
         "conn_fsm.go",
         "conn_io.go",
         "constraint.go",

pkg/sql/catalog/colinfo/result_columns.go

@@ -142,6 +142,11 @@ var ExplainPlanColumns = ResultColumns{
 	{Name: "info", Typ: types.String},
 }
 
+// ShowCommitTimestampColumns are the result columns of SHOW COMMIT TIMESTAMP.
+var ShowCommitTimestampColumns = ResultColumns{
+	{Name: "commit_timestamp", Typ: types.Decimal},
+}
+
 // ShowTraceColumns are the result columns of a SHOW [KV] TRACE statement.
 var ShowTraceColumns = ResultColumns{
 	{Name: "timestamp", Typ: types.TimestampTZ},

pkg/sql/conn_executor.go

@@ -1358,7 +1358,7 @@ type connExecutor struct {
 		txnFinishClosure struct {
 			// txnStartTime is the time that the transaction started.
 			txnStartTime time.Time
-			// implicit is whether or not the transaction was implicit.
+			// implicit is whether the transaction was implicit.
 			implicit bool
 		}
 
@@ -1414,6 +1414,13 @@ type connExecutor struct {
 		rowsWrittenLogged bool
 		rowsReadLogged    bool
 
+		// shouldAcceptReleaseCockroachRestartInCommitWait is set to true if the
+		// transaction had SAVEPOINT cockroach_restart installed at the time that
+		// SHOW COMMIT TIMESTAMP was executed to commit the transaction. If so, the
+		// connExecutor will permit one invocation of RELEASE SAVEPOINT
+		// cockroach_restart while in the CommitWait state.
+		shouldAcceptReleaseCockroachRestartInCommitWait bool
+
 		// hasAdminRole is used to cache if the user running the transaction
 		// has admin privilege. hasAdminRoleCache is set for the first statement
 		// in a transaction.
@@ -1542,6 +1549,11 @@ type connExecutor struct {
 	// totalActiveTimeStopWatch tracks the total active time of the session.
 	// This is defined as the time spent executing transactions and statements.
 	totalActiveTimeStopWatch *timeutil.StopWatch
+
+	// previousTransactionCommitTimestamp is the timestamp of the previous
+	// transaction which committed. It is zero-valued when there is a transaction
+	// open or the previous transaction did not successfully commit.
+	previousTransactionCommitTimestamp hlc.Timestamp
 }
 
 // ctxHolder contains a connection's context and, while session tracing is
@@ -1928,7 +1940,9 @@ func (ex *connExecutor) execCmd() error {
 			// The behavior is configurable, in case users want to preserve the
 			// behavior from v21.2 and earlier.
 			implicitTxnForBatch := ex.sessionData().EnableImplicitTransactionForBatchStatements
-			canAutoCommit := ex.implicitTxn() && (tcmd.LastInBatch || !implicitTxnForBatch)
+			canAutoCommit := ex.implicitTxn() &&
+				(tcmd.LastInBatchBeforeShowCommitTimestamp ||
+					tcmd.LastInBatch || !implicitTxnForBatch)
 			ev, payload, err = ex.execStmt(
 				ctx, tcmd.Statement, nil /* prepared */, nil /* pinfo */, stmtRes, canAutoCommit,
 			)

pkg/sql/conn_executor_exec.go

@@ -123,7 +123,7 @@ func (ex *connExecutor) execStmt(
 		// Note: when not using explicit transactions, we go through this transition
 		// for every statement. It is important to minimize the amount of work and
 		// allocations performed up to this point.
-		ev, payload = ex.execStmtInNoTxnState(ctx, ast)
+		ev, payload = ex.execStmtInNoTxnState(ctx, ast, res)
 
 	case stateOpen:
 		err = ex.execWithProfiling(ctx, ast, prepared, func(ctx context.Context) error {
@@ -590,6 +590,10 @@ func (ex *connExecutor) execStmtInOpenState(
 		ev, payload := ex.execRollbackToSavepointInOpenState(ctx, s, res)
 		return ev, payload, nil
 
+	case *tree.ShowCommitTimestamp:
+		ev, payload := ex.execShowCommitTimestampInOpenState(ctx, s, res, canAutoCommit)
+		return ev, payload, nil
+
 	case *tree.Prepare:
 		// This is handling the SQL statement "PREPARE". See execPrepare for
 		// handling of the protocol-level command for preparing statements.
@@ -777,6 +781,9 @@ func (ex *connExecutor) execStmtInOpenState(
 // the timestamps of the transaction accordingly.
 func (ex *connExecutor) handleAOST(ctx context.Context, stmt tree.Statement) error {
 	if _, isNoTxn := ex.machine.CurState().(stateNoTxn); isNoTxn {
+		if _, ok := stmt.(*tree.ShowCommitTimestamp); ok {
+			return nil
+		}
 		return errors.AssertionFailedf(
 			"cannot handle AOST clause without a transaction",
 		)
@@ -1670,7 +1677,7 @@ var eventStartExplicitTxn fsm.Event = eventTxnStart{ImplicitTxn: fsm.False}
 // the cursor is not advanced. This means that the statement will run again in
 // stateOpen, at each point its results will also be flushed.
 func (ex *connExecutor) execStmtInNoTxnState(
-	ctx context.Context, ast tree.Statement,
+	ctx context.Context, ast tree.Statement, res RestrictedCommandResult,
 ) (_ fsm.Event, payload fsm.EventPayload) {
 	switch s := ast.(type) {
 	case *tree.BeginTransaction:
@@ -1693,6 +1700,8 @@ func (ex *connExecutor) execStmtInNoTxnState(
 				historicalTs,
 				ex.transitionCtx,
 				ex.QualityOfService())
+	case *tree.ShowCommitTimestamp:
+		return ex.execShowCommitTimestampInNoTxnState(ctx, s, res)
 	case *tree.CommitTransaction, *tree.ReleaseSavepoint,
 		*tree.RollbackTransaction, *tree.SetTransaction, *tree.Savepoint:
 		return ex.makeErrEvent(errNoTransactionInProgress, ast)
@@ -1815,7 +1824,15 @@ func (ex *connExecutor) execStmtInCommitWaitState(
 			ex.incrementExecutedStmtCounter(ast)
 		}
 	}()
-	switch ast.(type) {
+	switch s := ast.(type) {
+	case *tree.ReleaseSavepoint:
+		if ex.extraTxnState.shouldAcceptReleaseCockroachRestartInCommitWait &&
+			s.Savepoint == commitOnReleaseSavepointName {
+			ex.extraTxnState.shouldAcceptReleaseCockroachRestartInCommitWait = false
+			return nil, nil
+		}
+	case *tree.ShowCommitTimestamp:
+		return ex.execShowCommitTimestampInCommitWaitState(ctx, s, res)
 	case *tree.CommitTransaction, *tree.RollbackTransaction:
 		// Reply to a rollback with the COMMIT tag, by analogy to what we do when we
 		// get a COMMIT in state Aborted.
@@ -1828,13 +1845,11 @@ func (ex *connExecutor) execStmtInCommitWaitState(
 				return nil
 			},
 		)
-	default:
-		ev = eventNonRetriableErr{IsCommit: fsm.False}
-		payload = eventNonRetriableErrPayload{
+	}
+	return eventNonRetriableErr{IsCommit: fsm.False},
+		eventNonRetriableErrPayload{
 			err: sqlerrors.NewTransactionCommittedError(),
 		}
-		return ev, payload
-	}
 }
 
 // runObserverStatement executes the given observer statement.
@@ -2257,6 +2272,8 @@ func (ex *connExecutor) onTxnFinish(ctx context.Context, ev txnEvent) {
 			}
 			ex.server.ServerMetrics.StatsMetrics.DiscardedStatsCount.Inc(1)
 		}
+		// If we have a commitTimestamp, we should use it.
+		ex.previousTransactionCommitTimestamp.Forward(ev.commitTimestamp)
 	}
 }
 
@@ -2320,6 +2337,8 @@ func (ex *connExecutor) recordTransactionStart(txnID uuid.UUID) {
 	ex.extraTxnState.shouldExecuteOnTxnRestart = true
 
 	ex.statsCollector.StartTransaction()
+
+	ex.previousTransactionCommitTimestamp = hlc.Timestamp{}
 }
 
 func (ex *connExecutor) recordTransactionFinish(

pkg/sql/conn_executor_prepare.go

@@ -40,7 +40,13 @@ func (ex *connExecutor) execPrepare(
 	// descriptors for type checking. This implicit txn will be open until
 	// the Sync message is handled.
 	if _, isNoTxn := ex.machine.CurState().(stateNoTxn); isNoTxn {
-		return ex.beginImplicitTxn(ctx, parseCmd.AST)
+		// The one exception is that we must not open a new transaction when
+		// preparing SHOW COMMIT TIMESTAMP. If we did, it would destroy the
+		// information about the previous transaction. We expect to execute
+		// this command in NoTxn.
+		if _, ok := parseCmd.AST.(*tree.ShowCommitTimestamp); !ok {
+			return ex.beginImplicitTxn(ctx, parseCmd.AST)
+		}
 	} else if _, isAbortedTxn := ex.machine.CurState().(stateAborted); isAbortedTxn {
 		if !ex.isAllowedInAbortedTxn(parseCmd.AST) {
 			return retErr(sqlerrors.NewTransactionAbortedError("" /* customMsg */))
@@ -315,7 +321,12 @@ func (ex *connExecutor) execBind(
 	// SQL EXECUTE command (which also needs to bind and resolve types) is
 	// handled separately in conn_executor_exec.
 	if _, isNoTxn := ex.machine.CurState().(stateNoTxn); isNoTxn {
-		return ex.beginImplicitTxn(ctx, ps.AST)
+		// The one critical exception is that we must not open a transaction when
+		// executing SHOW COMMIT TIMESTAMP as it would destroy the information
+		// about the previously committed transaction.
+		if _, ok := ps.AST.(*tree.ShowCommitTimestamp); !ok {
+			return ex.beginImplicitTxn(ctx, ps.AST)
+		}
 	} else if _, isAbortedTxn := ex.machine.CurState().(stateAborted); isAbortedTxn {
 		if !ex.isAllowedInAbortedTxn(ps.AST) {
 			return retErr(sqlerrors.NewTransactionAbortedError("" /* customMsg */))