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sessionctx: default paging size bytes to 4 MiB #69868

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sessionctx: default paging size bytes to 4 MiB #69868

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00b3f68
sessionctx: default paging size bytes to 4 MiB
JmPotato Jul 15, 2026
6920105
sessionctx: declare paging hint support inline
JmPotato Jul 15, 2026
cf002db
sessionctx: clarify paging size bytes scope
JmPotato Jul 15, 2026
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10 changes: 9 additions & 1 deletion pkg/sessionctx/vardef/tidb_vars.go
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Original file line number Diff line number Diff line change
Expand Up @@ -540,6 +540,8 @@ const (
TiDBMaxPagingSize = "tidb_max_paging_size"

// TiDBPagingSizeBytes is the byte budget per coprocessor page.
// A non-zero value takes effect only when Resource Control is enabled and the active Resource Group
// is non-burstable (has limited burst).
// 0 means disabled (no byte-budget paging).
TiDBPagingSizeBytes = "tidb_paging_size_bytes"

Expand Down Expand Up @@ -1517,7 +1519,13 @@ const (
DefInitChunkSize = 32
DefMinPagingSize = int(paging.MinPagingSize)
DefMaxPagingSize = int(paging.MinAllowedMaxPagingSize)
DefPagingSizeBytes = 0
// DefPagingSizeBytes defaults byte-budget paging to 4 MiB. It takes effect only when Resource Control
// is enabled and the active Resource Group is non-burstable (has limited burst).
// Regression tests across cap-bound and under-cap TPC-C, FullScan, and Join workloads found no
// material performance regression. Among the tested 1, 2, 4, 8, and 16 MiB candidates, 4 MiB
// provided the best overall balance of throughput, tail latency, RU stability, RPC overhead, CPU,
// and RU efficiency.
DefPagingSizeBytes = 4 * 1024 * 1024
DefMaxChunkSize = 1024
DefDMLBatchSize = 0
DefMaxPreparedStmtCount = -1
Expand Down
1 change: 0 additions & 1 deletion pkg/sessionctx/variable/setvar_affect.go
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Expand Up @@ -92,7 +92,6 @@ var isHintUpdatableVerified = map[string]struct{}{
"tidb_max_chunk_size": {},
"tidb_min_paging_size": {},
"tidb_max_paging_size": {},
"tidb_paging_size_bytes": {},
"tidb_enable_cascades_planner": {},
"tidb_merge_join_concurrency": {},
"tidb_index_merge_intersection_concurrency": {},
Expand Down
2 changes: 1 addition & 1 deletion pkg/sessionctx/variable/sysvar.go
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Expand Up @@ -3070,7 +3070,7 @@ var defaultSysVars = []*SysVar{
s.MaxPagingSize = tidbOptPositiveInt32(val, vardef.DefMaxPagingSize)
return nil
}},
{Scope: vardef.ScopeGlobal | vardef.ScopeSession, Name: vardef.TiDBPagingSizeBytes, Value: strconv.Itoa(vardef.DefPagingSizeBytes), Type: vardef.TypeUnsigned, MinValue: 0, MaxValue: math.MaxInt64, SetSession: func(s *SessionVars, val string) error {
{Scope: vardef.ScopeGlobal | vardef.ScopeSession, Name: vardef.TiDBPagingSizeBytes, Value: strconv.Itoa(vardef.DefPagingSizeBytes), Type: vardef.TypeUnsigned, MinValue: 0, MaxValue: math.MaxInt64, IsHintUpdatableVerified: true, SetSession: func(s *SessionVars, val string) error {
s.PagingSizeBytes = int(TidbOptInt64(val, int64(vardef.DefPagingSizeBytes)))
return nil
}},
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6 changes: 3 additions & 3 deletions tests/integrationtest/r/sessionctx/setvar.result
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Expand Up @@ -1102,11 +1102,11 @@ select /*+ set_var(tidb_paging_size_bytes=1048576) */ @@tidb_paging_size_bytes;
1048576
select @@tidb_paging_size_bytes;
@@tidb_paging_size_bytes
0
4194304
set @@tidb_paging_size_bytes=default;
select @@tidb_paging_size_bytes;
@@tidb_paging_size_bytes
0
4194304
set @@tidb_paging_size_bytes=0;
select @@tidb_paging_size_bytes;
@@tidb_paging_size_bytes
Expand All @@ -1118,7 +1118,7 @@ select @@tidb_paging_size_bytes;
set @@tidb_paging_size_bytes=default;
select @@tidb_paging_size_bytes;
@@tidb_paging_size_bytes
0
4194304
select /*+ set_var(tidb_enable_cascades_planner=0) */ @@tidb_enable_cascades_planner;
@@tidb_enable_cascades_planner
0
Expand Down
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