optimizer: add enable_less_reduce_in_eqprop feature flag

Author: ggevay

src/repr/src/optimize.rs

@@ -124,6 +124,8 @@ optimizer_feature_flags!({
     enable_projection_pushdown_after_relation_cse: bool,
     // See the feature flag of the same name.
     enable_let_prefix_extraction: bool,
+    // See the feature flag of the same name.
+    enable_less_reduce_in_eqprop: bool,
 });
 
 /// A trait used to implement layered config construction.

src/sql/src/plan/statement/ddl.rs

@@ -4654,7 +4654,9 @@ pub fn unplan_create_cluster(
                 enable_join_prioritize_arranged,
                 enable_projection_pushdown_after_relation_cse,
                 enable_let_prefix_extraction: _,
+                enable_less_reduce_in_eqprop: _,
             } = optimizer_feature_overrides;
+            // The ones from above that don't occur below are not wired up to cluster features.
             let features_extracted = ClusterFeatureExtracted {
                 // Seen is ignored when unplanning.
                 seen: Default::default(),

src/sql/src/plan/statement/dml.rs

@@ -435,6 +435,7 @@ impl TryFrom<ExplainPlanOptionExtracted> for ExplainConfig {
                 enable_projection_pushdown_after_relation_cse: v
                     .enable_projection_pushdown_after_relation_cse,
                 enable_let_prefix_extraction: Default::default(),
+                enable_less_reduce_in_eqprop: Default::default(),
             },
         })
     }

src/sql/src/session/vars/definitions.rs

@@ -2209,6 +2209,12 @@ feature_flags!(
         default: true,
         enable_for_item_parsing: false,
     },
+    {
+        name: enable_less_reduce_in_eqprop,
+        desc: "Run MSE::reduce in EquivalencePropagation only if reduce_expr changed something.",
+        default: true,
+        enable_for_item_parsing: false,
+    },
 );
 
 impl From<&super::SystemVars> for OptimizerFeatures {
@@ -2228,6 +2234,7 @@ impl From<&super::SystemVars> for OptimizerFeatures {
             enable_projection_pushdown_after_relation_cse: vars
                 .enable_projection_pushdown_after_relation_cse(),
             enable_let_prefix_extraction: vars.enable_let_prefix_extraction(),
+            enable_less_reduce_in_eqprop: vars.enable_less_reduce_in_eqprop(),
         }
     }
 }