Fix a minor EXPLAIN bug around count(*)

src/adapter/src/explain/insights.rs

@@ -14,10 +14,7 @@ use std::fmt::Debug;
 use std::sync::Arc;
 
 use mz_compute_types::dataflows::{BuildDesc, DataflowDescription};
-use mz_expr::{
-    AccessStrategy, AggregateExpr, AggregateFunc, Id, MirRelationExpr, OptimizedMirRelationExpr,
-    RowSetFinishing,
-};
+use mz_expr::{AccessStrategy, Id, MirRelationExpr, OptimizedMirRelationExpr, RowSetFinishing};
 use mz_ore::num::NonNeg;
 use mz_repr::explain::ExprHumanizer;
 use mz_repr::{GlobalId, Timestamp};
@@ -266,15 +263,7 @@ fn global_insights(
             let [aggregate] = aggregates.as_slice() else {
                 return;
             };
-            let AggregateExpr {
-                func: AggregateFunc::Count,
-                distinct: false,
-                expr,
-            } = aggregate
-            else {
-                return;
-            };
-            if !expr.is_literal_true() {
+            if !aggregate.is_count_asterisk() {
                 return;
             }
             let name = structured_name(humanizer, *id);

src/expr/src/relation.rs

@@ -3340,16 +3340,9 @@ impl AggregateExpr {
         match self {
             AggregateExpr {
                 func: AggregateFunc::Count,
-                expr:
-                    MirScalarExpr::Literal(
-                        Ok(row),
-                        mz_repr::ColumnType {
-                            scalar_type: mz_repr::ScalarType::Bool,
-                            nullable: false,
-                        },
-                    ),
-                ..
-            } => row.unpack_first() == mz_repr::Datum::True,
+                expr,
+                distinct: false,
+            } => expr.is_literal_true(),
             _ => false,
         }
     }

src/sql/src/func.rs

@@ -3046,6 +3046,8 @@ pub static PG_CATALOG_BUILTINS: LazyLock<BTreeMap<&'static str, Func>> = LazyLoc
         "count" => Aggregate {
             params!() => Operation::nullary(|_ecx| {
                 // COUNT(*) is equivalent to COUNT(true).
+                // This is mirrored in `AggregateExpr::is_count_asterisk`, so if you modify this,
+                // then attend to that code also.
                 Ok((HirScalarExpr::literal_true(), AggregateFunc::Count))
             }) => Int64, 2803;
             params!(Any) => AggregateFunc::Count => Int64, 2147;

test/sqllogictest/explain/optimized_plan_as_text.slt

@@ -1623,3 +1623,96 @@ Source materialize.public.t6
 Target cluster: no_replicas
 
 EOF
+
+# `count(*)` is planned as `count(true)`. We take care in EXPLAIN to show `count(true)` as `count(*)` to avoid confusing
+# users.
+query T multiline
+EXPLAIN OPTIMIZED PLAN AS TEXT FOR
+SELECT count(*)
+FROM t5;
+----
+Explained Query:
+  With
+    cte l0 =
+      Reduce aggregates=[count(*)]
+        Project ()
+          ReadStorage materialize.public.t5
+  Return
+    Union
+      Get l0
+      Map (0)
+        Union
+          Negate
+            Project ()
+              Get l0
+          Constant
+            - ()
+
+Source materialize.public.t5
+
+Target cluster: no_replicas
+
+EOF
+
+query error DISTINCT \* not supported as function args
+EXPLAIN OPTIMIZED PLAN AS TEXT FOR
+SELECT count(distinct *)
+FROM t5;
+
+# `count(true)` is currently also printed as `count(*)` in EXPLAIN, which I'd say is fine.
+query T multiline
+EXPLAIN OPTIMIZED PLAN AS TEXT FOR
+SELECT count(true)
+FROM t5;
+----
+Explained Query:
+  With
+    cte l0 =
+      Reduce aggregates=[count(*)]
+        Project ()
+          ReadStorage materialize.public.t5
+  Return
+    Union
+      Get l0
+      Map (0)
+        Union
+          Negate
+            Project ()
+              Get l0
+          Constant
+            - ()
+
+Source materialize.public.t5
+
+Target cluster: no_replicas
+
+EOF
+
+# But `count(DISTINCT true)` means an entirely different thing, so EXPLAIN shouldn't conflate it with `count(*)`.
+query T multiline
+EXPLAIN OPTIMIZED PLAN AS TEXT FOR
+SELECT count(DISTINCT true)
+FROM t5;
+----
+Explained Query:
+  With
+    cte l0 =
+      Reduce aggregates=[count(distinct true)]
+        Project ()
+          ReadStorage materialize.public.t5
+  Return
+    Union
+      Get l0
+      Map (0)
+        Union
+          Negate
+            Project ()
+              Get l0
+          Constant
+            - ()
+
+Source materialize.public.t5
+
+Target cluster: no_replicas
+
+EOF