sql: Let parser determine if query is empty

Fixes cockroachdb#3852

sql/executor.go

@@ -85,6 +85,9 @@ type Response struct {
 	// The list of results. There is one result object per SQL statement in the
 	// request.
 	Results []Result
+
+	// Indicates that after parsing, the request contained 0 non-empty statements.
+	Empty bool
 }
 
 // Result corresponds to the execution of a single SQL statement.
@@ -337,6 +340,11 @@ func (e *Executor) execStmts(sql string, planMaker *planner) Response {
 		resp.Results = append(resp.Results, makeResultFromError(planMaker, roachpb.NewError(err)))
 		return resp
 	}
+	if len(stmts) == 0 {
+		resp.Empty = true
+		return resp
+	}
+
 	for _, stmt := range stmts {
 		result, err := e.execStmt(stmt, planMaker)
 		if err != nil {

sql/pgwire/v3.go

@@ -23,7 +23,6 @@ import (
 	"net"
 	"reflect"
 	"strconv"
-	"strings"
 
 	"github.com/lib/pq/oid"
 
@@ -553,13 +552,6 @@ func (c *v3Conn) executeStatements(stmts string, params []parser.Datum, formatCo
 
 	c.session.Database = c.opts.database
 
-	// TODO(dt): this is a clumsy check better left to the actual parser. #3852
-	if len(strings.TrimSpace(stmts)) == 0 {
-		// Skip executor and just send EmptyQueryResponse.
-		c.writeBuf.initMsg(serverMsgEmptyQuery)
-		return c.writeBuf.finishMsg(c.wr)
-	}
-
 	resp, _, err := c.executor.ExecuteStatements(c.opts.user, c.session, stmts, params)
 	if err != nil {
 		return c.sendError(err.Error())
@@ -570,6 +562,11 @@ func (c *v3Conn) executeStatements(stmts string, params []parser.Datum, formatCo
 
 	c.opts.database = c.session.Database
 	tracing.AnnotateTrace()
+	if resp.Empty {
+		// Skip executor and just send EmptyQueryResponse.
+		c.writeBuf.initMsg(serverMsgEmptyQuery)
+		return c.writeBuf.finishMsg(c.wr)
+	}
 	return c.sendResponse(resp, formatCodes, sendDescription, limit)
 }
 

sql/pgwire_test.go

@@ -556,7 +556,7 @@ func TestCmdCompleteVsEmptyStatements(t *testing.T) {
 	}
 	_, _ = nonempty.RowsAffected() // should not panic if lib/pq returned a non-nil result.
 
-	empty, err := db.Exec(" ")
+	empty, err := db.Exec(" ; ; ;")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -565,7 +565,6 @@ func TestCmdCompleteVsEmptyStatements(t *testing.T) {
 	}()
 	_, _ = empty.RowsAffected() // should panic if lib/pq returned a nil result as expected.
 	t.Fatal("should not get here -- empty result from empty query should panic first")
-	// TODO(dt): clean this up with testify/assert and add tests for less trivial empty queries.
 }
 
 // Unfortunately lib/pq doesn't expose returned command tags directly, but we can test