cockroachdb · mgartner · Nov 3, 2025 · Oct 31, 2025
diff --git a/pkg/sql/exec_util.go b/pkg/sql/exec_util.go
@@ -4479,6 +4479,10 @@ func (m *sessionDataMutator) SetOptimizerClampInequalitySelectivity(val bool) {
 	m.data.OptimizerClampInequalitySelectivity = val
 }
 
+func (m *sessionDataMutator) SetOptimizerUseMaxFrequencySelectivity(val bool) {
+	m.data.OptimizerUseMaxFrequencySelectivity = val
+}
+
 // Utility functions related to scrubbing sensitive information on SQL Stats.
 
 // quantizeCounts ensures that the Count field in the

diff --git a/pkg/sql/logictest/testdata/logic_test/information_schema b/pkg/sql/logictest/testdata/logic_test/information_schema
@@ -4218,6 +4218,7 @@ optimizer_use_improved_zigzag_join_costing                       on
 optimizer_use_limit_ordering_for_streaming_group_by              on
 optimizer_use_lock_elision_multiple_families                     off
 optimizer_use_lock_op_for_serializable                           off
+optimizer_use_max_frequency_selectivity                          on
 optimizer_use_merged_partial_statistics                          on
 optimizer_use_multicol_stats                                     on
 optimizer_use_not_visible_indexes                                off

diff --git a/pkg/sql/logictest/testdata/logic_test/pg_catalog b/pkg/sql/logictest/testdata/logic_test/pg_catalog
@@ -3120,6 +3120,7 @@ optimizer_use_improved_zigzag_join_costing                       on
 optimizer_use_limit_ordering_for_streaming_group_by              on                  NULL      NULL        NULL        string
 optimizer_use_lock_elision_multiple_families                     off                 NULL      NULL        NULL        string
 optimizer_use_lock_op_for_serializable                           off                 NULL      NULL        NULL        string
+optimizer_use_max_frequency_selectivity                          on                  NULL      NULL        NULL        string
 optimizer_use_merged_partial_statistics                          on                  NULL      NULL        NULL        string
 optimizer_use_multicol_stats                                     on                  NULL      NULL        NULL        string
 optimizer_use_not_visible_indexes                                off                 NULL      NULL        NULL        string
@@ -3367,6 +3368,7 @@ optimizer_use_improved_zigzag_join_costing                       on
 optimizer_use_limit_ordering_for_streaming_group_by              on                  NULL  user     NULL      on                  on
 optimizer_use_lock_elision_multiple_families                     off                 NULL  user     NULL      off                 off
 optimizer_use_lock_op_for_serializable                           off                 NULL  user     NULL      off                 off
+optimizer_use_max_frequency_selectivity                          on                  NULL  user     NULL      on                  on
 optimizer_use_merged_partial_statistics                          on                  NULL  user     NULL      on                  on
 optimizer_use_multicol_stats                                     on                  NULL  user     NULL      on                  on
 optimizer_use_not_visible_indexes                                off                 NULL  user     NULL      off                 off
@@ -3605,6 +3607,7 @@ optimizer_use_improved_zigzag_join_costing                       NULL    NULL
 optimizer_use_limit_ordering_for_streaming_group_by              NULL    NULL     NULL     NULL        NULL
 optimizer_use_lock_elision_multiple_families                     NULL    NULL     NULL     NULL        NULL
 optimizer_use_lock_op_for_serializable                           NULL    NULL     NULL     NULL        NULL
+optimizer_use_max_frequency_selectivity                          NULL    NULL     NULL     NULL        NULL
 optimizer_use_merged_partial_statistics                          NULL    NULL     NULL     NULL        NULL
 optimizer_use_multicol_stats                                     NULL    NULL     NULL     NULL        NULL
 optimizer_use_not_visible_indexes                                NULL    NULL     NULL     NULL        NULL

diff --git a/pkg/sql/logictest/testdata/logic_test/show_source b/pkg/sql/logictest/testdata/logic_test/show_source
@@ -184,6 +184,7 @@ optimizer_use_improved_zigzag_join_costing                       on
 optimizer_use_limit_ordering_for_streaming_group_by              on
 optimizer_use_lock_elision_multiple_families                     off
 optimizer_use_lock_op_for_serializable                           off
+optimizer_use_max_frequency_selectivity                          on
 optimizer_use_merged_partial_statistics                          on
 optimizer_use_multicol_stats                                     on
 optimizer_use_not_visible_indexes                                off

@@ -213,6 +213,7 @@ type Memo struct {
 	useImprovedHoistJoinProject                bool
 	clampLowHistogramSelectivity               bool
 	clampInequalitySelectivity                 bool
+	useMaxFrequencySelectivity                 bool
 
 	// txnIsoLevel is the isolation level under which the plan was created. This
 	// affects the planning of some locking operations, so it must be included in
@@ -326,6 +327,7 @@ func (m *Memo) Init(ctx context.Context, evalCtx *eval.Context) {
 		useImprovedHoistJoinProject:                evalCtx.SessionData().OptimizerUseImprovedHoistJoinProject,
 		clampLowHistogramSelectivity:               evalCtx.SessionData().OptimizerClampLowHistogramSelectivity,
 		clampInequalitySelectivity:                 evalCtx.SessionData().OptimizerClampInequalitySelectivity,
+		useMaxFrequencySelectivity:                 evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity,
 		txnIsoLevel:                                evalCtx.TxnIsoLevel,
 	}
 	m.metadata.Init()
@@ -503,6 +505,7 @@ func (m *Memo) IsStale(
 		m.useImprovedHoistJoinProject != evalCtx.SessionData().OptimizerUseImprovedHoistJoinProject ||
 		m.clampLowHistogramSelectivity != evalCtx.SessionData().OptimizerClampLowHistogramSelectivity ||
 		m.clampInequalitySelectivity != evalCtx.SessionData().OptimizerClampInequalitySelectivity ||
+		m.useMaxFrequencySelectivity != evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity ||
 		m.txnIsoLevel != evalCtx.TxnIsoLevel {
 		return true, nil
 	}

@@ -510,6 +510,12 @@ func TestMemoIsStale(t *testing.T) {
 	evalCtx.SessionData().OptimizerUseImprovedMultiColumnSelectivityEstimate = false
 	notStale()
 
+	// Stale optimizer_use_max_frequency_selectivity.
+	evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity = true
+	stale()
+	evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity = false
+	notStale()
+
 	// Stale optimizer_prove_implication_with_virtual_computed_columns.
 	evalCtx.SessionData().OptimizerProveImplicationWithVirtualComputedColumns = true
 	stale()

@@ -4670,6 +4670,9 @@ func (sb *statisticsBuilder) selectivityFromMaxFrequencies(
 ) (selectivity, selectivityUpperBound props.Selectivity, maxFreqCols opt.ColSet) {
 	selectivity = props.OneSelectivity
 	selectivityUpperBound = props.OneSelectivity
+	if !sb.evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity {
+		return selectivity, selectivityUpperBound, opt.ColSet{}
+	}
 	for col, ok := cols.Next(0); ok; col, ok = cols.Next(col + 1) {
 		c := opt.MakeColSet(col)
 		inputColStat, inputStats := sb.colStatFromInput(c, e)

@@ -254,6 +254,27 @@ select
  └── filters
       └── i:2 = $1 [type=bool, outer=(2), constraints=(/2: (/NULL - ]), fd=()-->(2)]
 
+# Fallback to ~row_count/distinct_count if
+# optimizer_use_max_frequency_selectivity is false.
+norm set=(optimizer_use_max_frequency_selectivity=false)
+SELECT * FROM t WHERE i = $1
+----
+select
+ ├── columns: k:1(int!null) i:2(int!null) s:3(string)
+ ├── has-placeholder
+ ├── stats: [rows=24.25, distinct(2)=1, null(2)=0]
+ ├── key: (1)
+ ├── fd: ()-->(2), (1)-->(3)
+ ├── scan t
+ │    ├── columns: k:1(int!null) i:2(int) s:3(string)
+ │    ├── stats: [rows=1000, distinct(1)=1000, null(1)=0, distinct(2)=41, null(2)=30]
+ │    │   histogram(2)=  0   30   0  0  90  10   180  10   270  20   360  30
+ │    │                <--- NULL --- 0 ---- 100 ----- 200 ----- 300 ----- 400
+ │    ├── key: (1)
+ │    └── fd: (1)-->(2,3)
+ └── filters
+      └── i:2 = $1 [type=bool, outer=(2), constraints=(/2: (/NULL - ]), fd=()-->(2)]
+
 # Similar case as above, but with opt to ensure the correct row counts are used
 # for new memo groups.
 opt

diff --git a/pkg/sql/sessiondatapb/local_only_session_data.proto b/pkg/sql/sessiondatapb/local_only_session_data.proto
@@ -749,6 +749,9 @@ message LocalOnlySessionData {
   // clamp the selectivity of open-ended inequality filters (e.g. <, >, !=)
   // but not (=, BETWEEN etc.) to a minimum threshold.
   bool optimizer_clamp_inequality_selectivity = 190;
+  // OptimizerUseMaxFrequencySelectivity, when true, indicates that the
+  // optimizer should use max frequency for selectivity estimation.
+  bool optimizer_use_max_frequency_selectivity = 191;
 
   ///////////////////////////////////////////////////////////////////////////
   // WARNING: consider whether a session parameter you're adding needs to  //

diff --git a/pkg/sql/vars.go b/pkg/sql/vars.go
@@ -3723,6 +3723,23 @@ var varGen = map[string]sessionVar{
 		GlobalDefault: globalTrue,
 	},
 
+	// CockroachDB extension.
+	`optimizer_use_max_frequency_selectivity`: {
+		GetStringVal: makePostgresBoolGetStringValFn(`optimizer_use_max_frequency_selectivity`),
+		Set: func(_ context.Context, m sessionDataMutator, s string) error {
+			b, err := paramparse.ParseBoolVar("optimizer_use_max_frequency_selectivity", s)
+			if err != nil {
+				return err
+			}
+			m.SetOptimizerUseMaxFrequencySelectivity(b)
+			return nil
+		},
+		Get: func(evalCtx *extendedEvalContext, _ *kv.Txn) (string, error) {
+			return formatBoolAsPostgresSetting(evalCtx.SessionData().OptimizerUseMaxFrequencySelectivity), nil
+		},
+		GlobalDefault: globalTrue,
+	},
+
 	// CockroachDB extension.
 	`optimizer_prove_implication_with_virtual_computed_columns`: {
 		GetStringVal: makePostgresBoolGetStringValFn(`optimizer_prove_implication_with_virtual_computed_columns`),