distsql: do not change concurrency for keep order request when @@tidb_distsql_scan_concurrency is set (#60803)

close #60891

Author: tiancaiamao

pkg/distsql/request_builder.go

@@ -32,6 +32,7 @@ import (
 	"github.com/pingcap/tidb/pkg/kv"
 	"github.com/pingcap/tidb/pkg/meta/model"
 	"github.com/pingcap/tidb/pkg/parser/mysql"
+	"github.com/pingcap/tidb/pkg/sessionctx/vardef"
 	"github.com/pingcap/tidb/pkg/tablecodec"
 	"github.com/pingcap/tidb/pkg/types"
 	"github.com/pingcap/tidb/pkg/util/codec"
@@ -81,14 +82,17 @@ func (builder *RequestBuilder) Build() (*kv.Request, error) {
 
 	if dag := builder.dag; dag != nil {
 		if execCnt := len(dag.Executors); execCnt == 1 {
-			oldConcurrency := builder.Request.Concurrency
 			// select * from t order by id
-			if builder.Request.KeepOrder {
+			if builder.Request.KeepOrder && builder.Request.Concurrency == vardef.DefDistSQLScanConcurrency {
 				// When the DAG is just simple scan and keep order, set concurrency to 2.
 				// If a lot data are returned to client, mysql protocol is the bottleneck so concurrency 2 is enough.
 				// If very few data are returned to client, the speed is not optimal but good enough.
+				//
+				// If a user set @@tidb_distsql_scan_concurrency, he must be doing it by intention.
+				// so only rewrite concurrency when @@tidb_distsql_scan_concurrency is default value.
 				switch dag.Executors[0].Tp {
 				case tipb.ExecType_TypeTableScan, tipb.ExecType_TypeIndexScan, tipb.ExecType_TypePartitionTableScan:
+					oldConcurrency := builder.Request.Concurrency
 					builder.Request.Concurrency = 2
 					failpoint.Inject("testRateLimitActionMockConsumeAndAssert", func(val failpoint.Value) {
 						if val.(bool) {

tests/integrationtest/r/executor/issues.result

@@ -1000,6 +1000,14 @@ Limit_11	1.00	<actRows>	root	NULL	NULL	offset:0, count:100	<memory>	<disk>
   └─Limit_19	1.00	<actRows>	cop[tikv]	NULL	NULL	offset:0, count:100	<memory>	<disk>
     └─Selection_18	1.00	<actRows>	cop[tikv]	NULL	NULL	eq(executor__issues.pt.val, 126)	<memory>	<disk>
       └─TableFullScan_17	256.00	<actRows>	cop[tikv]	table:pt	NULL	keep order:true	<memory>	<disk>
+explain analyze select /*+ set_var(tidb_distsql_scan_concurrency=5)*/ * from t order by id;
+id	estRows	actRows	task	access object	execution info	operator info	memory	disk
+TableReader_11	256.00	<actRows>	root	NULL	max_distsql_concurrency: 5	NULL	<memory>	<disk>
+└─TableFullScan_10	256.00	<actRows>	cop[tikv]	table:t	NULL	keep order:true	<memory>	<disk>
+explain analyze select /*+ set_var(tidb_distsql_scan_concurrency=15)*/ * from t order by id;
+id	estRows	actRows	task	access object	execution info	operator info	memory	disk
+TableReader_11	256.00	<actRows>	root	NULL	max_distsql_concurrency: 2	NULL	<memory>	<disk>
+└─TableFullScan_10	256.00	<actRows>	cop[tikv]	table:t	NULL	keep order:true	<memory>	<disk>
 CREATE TABLE test_55837 (col1 int(4) NOT NULL, col2 bigint(4) NOT NULL, KEY col2_index (col2));
 insert into test_55837 values(0,1725292800),(0,1725292800);
 select from_unixtime( if(col2 >9999999999, col2/1000, col2), '%Y-%m-%d %H:%i:%s') as result from test_55837;

tests/integrationtest/t/executor/issues.test

@@ -763,6 +763,15 @@ explain analyze select * from pt order by id limit 100000;      # expected dists
 -- replace_regex /.*max_distsql_concurrency: (?P<num>[0-9]+).*/max_distsql_concurrency: $num/ /tikv_task:.*// /time:.*// /data:.*//
 explain analyze select * from pt where val = 126 order by id limit 100;  # expected distsql concurrency 15
 
+-- replace_column 8 <memory> 9 <disk> 3 <actRows>
+-- replace_regex /.*max_distsql_concurrency: (?P<num>[0-9]+).*/max_distsql_concurrency: $num/ /tikv_task:.*// /time:.*// /data:.*//
+explain analyze select /*+ set_var(tidb_distsql_scan_concurrency=5)*/ * from t order by id;  # expected distsql concurrency 5
+
+-- replace_column 8 <memory> 9 <disk> 3 <actRows>
+-- replace_regex /.*max_distsql_concurrency: (?P<num>[0-9]+).*/max_distsql_concurrency: $num/ /tikv_task:.*// /time:.*// /data:.*//
+explain analyze select /*+ set_var(tidb_distsql_scan_concurrency=15)*/ * from t order by id;  # expected distsql concurrency 2
+
+
 # TestIssue55837
 CREATE TABLE test_55837 (col1 int(4) NOT NULL, col2 bigint(4) NOT NULL, KEY col2_index (col2));
 insert into test_55837 values(0,1725292800),(0,1725292800);