v3: instant-commit for autocommit

If we are in autocommit mode and vtgate does not break a DML into
smaller parts, then it has the opportunity to send that statement
through to a vttablet as autocommit in a single round-trip.

Reviewer instructions:
@demmer: I'm not too sure about the vtexplain fixes, or if additional
tests are required there. Extra scrutiny may be required there.

Implementation notes:
* SafeSession has a state machine for tracking autocommit state.
* The autocommit state is initialized by executor as needed.
* VCursor API has been changed for ExecMultiShard. It now accepts
  an extra canCommit flag that should be set to true if the engine
  is executing its final DML. This, combined with the autocommit
  state will decide if an instant autocommit is possible.

data/test/vtexplain/multi-output/deletesharded-output.txt

@@ -3,15 +3,15 @@ delete from music_extra where id=1
 
 1 ks_sharded/-40: begin
 1 ks_sharded/-40: delete from music_extra where id in (1) /* vtgate:: keyspace_id:166b40b44aba4bd6 */
-2 ks_sharded/-40: commit
+1 ks_sharded/-40: commit
 
 ----------------------------------------------------------------------
 delete from music_extra where id=1 and extra='abc'
 
 1 ks_sharded/-40: begin
 1 ks_sharded/-40: select id from music_extra where id = 1 and extra = 'abc' limit 10001 for update /* vtgate:: keyspace_id:166b40b44aba4bd6 */
 1 ks_sharded/-40: delete from music_extra where id in (1) /* vtgate:: keyspace_id:166b40b44aba4bd6 */
-2 ks_sharded/-40: commit
+1 ks_sharded/-40: commit
 
 ----------------------------------------------------------------------
 delete from user where id=1

data/test/vtexplain/multi-output/unsharded-output.txt

@@ -8,36 +8,36 @@ insert into t1 (id,intval,floatval) values (1,2,3.14)
 
 1 ks_unsharded/-: begin
 1 ks_unsharded/-: insert into t1(id, intval, floatval) values (1, 2, 3.14)
-2 ks_unsharded/-: commit
+1 ks_unsharded/-: commit
 
 ----------------------------------------------------------------------
 update t1 set intval = 10
 
 1 ks_unsharded/-: begin
 1 ks_unsharded/-: select id from t1 limit 10001 for update
 1 ks_unsharded/-: update t1 set intval = 10 where id in (1)
-2 ks_unsharded/-: commit
+1 ks_unsharded/-: commit
 
 ----------------------------------------------------------------------
 update t1 set floatval = 9.99
 
 1 ks_unsharded/-: begin
 1 ks_unsharded/-: select id from t1 limit 10001 for update
 1 ks_unsharded/-: update t1 set floatval = 9.99 where id in (1)
-2 ks_unsharded/-: commit
+1 ks_unsharded/-: commit
 
 ----------------------------------------------------------------------
 delete from t1 where id = 100
 
 1 ks_unsharded/-: begin
 1 ks_unsharded/-: delete from t1 where id in (100)
-2 ks_unsharded/-: commit
+1 ks_unsharded/-: commit
 
 ----------------------------------------------------------------------
 insert into t1 (id,intval,floatval) values (1,2,3.14) on duplicate key update intval=3, floatval=3.14
 
 1 ks_unsharded/-: begin
 1 ks_unsharded/-: insert into t1(id, intval, floatval) values (1, 2, 3.14) on duplicate key update intval = 3, floatval = 3.14
-2 ks_unsharded/-: commit
+1 ks_unsharded/-: commit
 
 ----------------------------------------------------------------------

data/test/vtexplain/multi-output/updatesharded-output.txt

@@ -3,7 +3,7 @@ update user set nickname='alice' where id=1
 
 1 ks_sharded/-40: begin
 1 ks_sharded/-40: update user set nickname = 'alice' where id in (1) /* vtgate:: keyspace_id:166b40b44aba4bd6 */
-2 ks_sharded/-40: commit
+1 ks_sharded/-40: commit
 
 ----------------------------------------------------------------------
 update user set nickname='alice' where name='alice'
@@ -21,7 +21,7 @@ update user set pet='fido' where id=1
 
 1 ks_sharded/-40: begin
 1 ks_sharded/-40: update user set pet = 'fido' where id in (1) /* vtgate:: keyspace_id:166b40b44aba4bd6 */
-2 ks_sharded/-40: commit
+1 ks_sharded/-40: commit
 
 ----------------------------------------------------------------------
 update user set name='alicia' where id=1

go/vt/vtexplain/vtexplain_vttablet.go

@@ -216,6 +216,26 @@ func (t *explainTablet) ReadTransaction(ctx context.Context, target *querypb.Tar
  return t.tsv.ReadTransaction(ctx, target, dtid)
 }
 
+// ExecuteBatch is part of the QueryService interface.
+func (t *explainTablet) ExecuteBatch(ctx context.Context, target *querypb.Target, queries []*querypb.BoundQuery, asTransaction bool, transactionID int64, options *querypb.ExecuteOptions) ([]sqltypes.Result, error) {
+ t.mu.Lock()
+ t.currentTime = batchTime.Wait()
+
+ // Since the query is simulated being "sent" over the wire we need to
+ // copy the bindVars into the executor to avoid a data race.
+ for _, query := range queries {
+ bindVariables := sqltypes.CopyBindVariables(query.BindVariables)
+ t.tabletQueries = append(t.tabletQueries, &TabletQuery{
+ Time: t.currentTime,
+ SQL: query.Sql,
+ BindVars: bindVariables,
+ })
+ }
+ t.mu.Unlock()
+
+ return t.tsv.ExecuteBatch(ctx, target, queries, asTransaction, transactionID, options)
+}
+
 // BeginExecute is part of the QueryService interface.
 func (t *explainTablet) BeginExecute(ctx context.Context, target *querypb.Target, sql string, bindVariables map[string]*querypb.BindVariable, options *querypb.ExecuteOptions) (*sqltypes.Result, int64, error) {
  t.mu.Lock()