Add and adjust tests

Author: Algunenano

tests/queries/0_stateless/00172_early_constant_folding.reference

@@ -8,11 +8,10 @@ ExpressionTransform × 10
         (ReadFromPreparedSource)
         SourceFromSingleChunk 0 → 1
 (Expression)
-ExpressionTransform × 10
+ExpressionTransform
   (MergingAggregated)
-  Resize 1 → 10
-    MergingAggregatedTransform
-      (Expression)
-      ExpressionTransform
-        (ReadFromPreparedSource)
-        SourceFromSingleChunk 0 → 1
+  MergingAggregatedTransform
+    (Expression)
+    ExpressionTransform
+      (ReadFromPreparedSource)
+      SourceFromSingleChunk 0 → 1

tests/queries/0_stateless/03448_trivial_count_single_threaded_merge.reference

@@ -0,0 +1,55 @@
+-- { echo On }
+-- We should use just a single thread to merge the state of trivial count
+EXPLAIN PIPELINE SELECT count() FROM trivial_count;
+(Expression)
+ExpressionTransform
+  (MergingAggregated)
+  MergingAggregatedTransform
+    (Expression)
+    ExpressionTransform
+      (ReadFromPreparedSource)
+      SourceFromSingleChunk 0 → 1
+-- But not if we are filtering or doing other operations (no trivial count)
+EXPLAIN PIPELINE SELECT count() FROM trivial_count WHERE number % 3 = 2;
+(Expression)
+ExpressionTransform × 4
+  (Aggregating)
+  Resize 1 → 4
+    AggregatingTransform
+      (Expression)
+      ExpressionTransform
+        (Filter)
+        FilterTransform
+          (ReadFromMergeTree)
+          MergeTreeSelect(pool: ReadPoolInOrder, algorithm: InOrder) 0 → 1
+EXPLAIN PIPELINE SELECT count() FROM trivial_count GROUP BY number % 10;
+(Expression)
+ExpressionTransform × 4
+  (Aggregating)
+  Resize 1 → 4
+    AggregatingTransform
+      (Expression)
+      ExpressionTransform
+        (ReadFromMergeTree)
+        MergeTreeSelect(pool: ReadPoolInOrder, algorithm: InOrder) 0 → 1
+-- Other aggregations should still use as many threads as necessary
+EXPLAIN PIPELINE SELECT sum(number) FROM trivial_count;
+(Expression)
+ExpressionTransform × 4
+  (Aggregating)
+  Resize 1 → 4
+    AggregatingTransform
+      (Expression)
+      ExpressionTransform
+        (ReadFromMergeTree)
+        MergeTreeSelect(pool: ReadPoolInOrder, algorithm: InOrder) 0 → 1
+EXPLAIN PIPELINE SELECT count(), sum(number) FROM trivial_count;
+(Expression)
+ExpressionTransform × 4
+  (Aggregating)
+  Resize 1 → 4
+    AggregatingTransform
+      (Expression)
+      ExpressionTransform
+        (ReadFromMergeTree)
+        MergeTreeSelect(pool: ReadPoolInOrder, algorithm: InOrder) 0 → 1

tests/queries/0_stateless/03448_trivial_count_single_threaded_merge.sql

@@ -0,0 +1,17 @@
+SET enable_analyzer = 1;
+SET max_threads=4;
+
+DROP TABLE IF EXISTS trivial_count;
+CREATE TABLE trivial_count ORDER BY number AS Select * from numbers(10);
+
+-- { echo On }
+-- We should use just a single thread to merge the state of trivial count
+EXPLAIN PIPELINE SELECT count() FROM trivial_count;
+
+-- But not if we are filtering or doing other operations (no trivial count)
+EXPLAIN PIPELINE SELECT count() FROM trivial_count WHERE number % 3 = 2;
+EXPLAIN PIPELINE SELECT count() FROM trivial_count GROUP BY number % 10;
+
+-- Other aggregations should still use as many threads as necessary
+EXPLAIN PIPELINE SELECT sum(number) FROM trivial_count;
+EXPLAIN PIPELINE SELECT count(), sum(number) FROM trivial_count;