Create memory context for HashAgg with a reasonable maxBlockSize.

If the memory context's maxBlockSize is too big, a single block
allocation can suddenly exceed work_mem. For Hash Aggregation, this
can mean spilling to disk too early or reporting a confusing memory
usage number for EXPLAN ANALYZE.

Introduce CreateWorkExprContext(), which is like CreateExprContext(),
except that it creates the AllocSet with a maxBlockSize that is
reasonable in proportion to work_mem.

Right now, CreateWorkExprContext() is only used by Hash Aggregation,
but it may be generally useful in the future.

Discussion: https://postgr.es/m/412a3fbf306f84d8d78c4009e11791867e62b87c.camel@j-davis.com

src/backend/executor/execUtils.c

@@ -53,6 +53,7 @@
 #include "executor/executor.h"
 #include "jit/jit.h"
 #include "mb/pg_wchar.h"
+#include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
 #include "parser/parsetree.h"
 #include "partitioning/partdesc.h"
@@ -227,21 +228,13 @@ FreeExecutorState(EState *estate)
 	MemoryContextDelete(estate->es_query_cxt);
 }
 
-/* ----------------
- *		CreateExprContext
- *
- *		Create a context for expression evaluation within an EState.
- *
- * An executor run may require multiple ExprContexts (we usually make one
- * for each Plan node, and a separate one for per-output-tuple processing
- * such as constraint checking).  Each ExprContext has its own "per-tuple"
- * memory context.
- *
- * Note we make no assumption about the caller's memory context.
- * ----------------
+/*
+ * Internal implementation for CreateExprContext() and CreateWorkExprContext()
+ * that allows control over the AllocSet parameters.
  */
-ExprContext *
-CreateExprContext(EState *estate)
+static ExprContext *
+CreateExprContextInternal(EState *estate, Size minContextSize,
+						  Size initBlockSize, Size maxBlockSize)
 {
 	ExprContext *econtext;
 	MemoryContext oldcontext;
@@ -264,7 +257,9 @@ CreateExprContext(EState *estate)
 	econtext->ecxt_per_tuple_memory =
 		AllocSetContextCreate(estate->es_query_cxt,
 							  "ExprContext",
-							  ALLOCSET_DEFAULT_SIZES);
+							  minContextSize,
+							  initBlockSize,
+							  maxBlockSize);
 
 	econtext->ecxt_param_exec_vals = estate->es_param_exec_vals;
 	econtext->ecxt_param_list_info = estate->es_param_list_info;
@@ -294,6 +289,52 @@ CreateExprContext(EState *estate)
 	return econtext;
 }
 
+/* ----------------
+ *		CreateExprContext
+ *
+ *		Create a context for expression evaluation within an EState.
+ *
+ * An executor run may require multiple ExprContexts (we usually make one
+ * for each Plan node, and a separate one for per-output-tuple processing
+ * such as constraint checking).  Each ExprContext has its own "per-tuple"
+ * memory context.
+ *
+ * Note we make no assumption about the caller's memory context.
+ * ----------------
+ */
+ExprContext *
+CreateExprContext(EState *estate)
+{
+	return CreateExprContextInternal(estate, ALLOCSET_DEFAULT_SIZES);
+}
+
+
+/* ----------------
+ *		CreateWorkExprContext
+ *
+ * Like CreateExprContext, but specifies the AllocSet sizes to be reasonable
+ * in proportion to work_mem. If the maximum block allocation size is too
+ * large, it's easy to skip right past work_mem with a single allocation.
+ * ----------------
+ */
+ExprContext *
+CreateWorkExprContext(EState *estate)
+{
+	Size minContextSize = ALLOCSET_DEFAULT_MINSIZE;
+	Size initBlockSize = ALLOCSET_DEFAULT_INITSIZE;
+	Size maxBlockSize = ALLOCSET_DEFAULT_MAXSIZE;
+
+	/* choose the maxBlockSize to be no larger than 1/16 of work_mem */
+	while (16 * maxBlockSize > work_mem * 1024L)
+		maxBlockSize >>= 1;
+
+	if (maxBlockSize < ALLOCSET_DEFAULT_INITSIZE)
+		maxBlockSize = ALLOCSET_DEFAULT_INITSIZE;
+
+	return CreateExprContextInternal(estate, minContextSize,
+									 initBlockSize, maxBlockSize);
+}
+
 /* ----------------
  *		CreateStandaloneExprContext
  *

src/backend/executor/nodeAgg.c

@@ -3277,10 +3277,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 	}
 
 	if (use_hashing)
-	{
-		ExecAssignExprContext(estate, &aggstate->ss.ps);
-		aggstate->hashcontext = aggstate->ss.ps.ps_ExprContext;
-	}
+		aggstate->hashcontext = CreateWorkExprContext(estate);
 
 	ExecAssignExprContext(estate, &aggstate->ss.ps);
 

src/include/executor/executor.h

@@ -493,6 +493,7 @@ extern void end_tup_output(TupOutputState *tstate);
 extern EState *CreateExecutorState(void);
 extern void FreeExecutorState(EState *estate);
 extern ExprContext *CreateExprContext(EState *estate);
+extern ExprContext *CreateWorkExprContext(EState *estate);
 extern ExprContext *CreateStandaloneExprContext(void);
 extern void FreeExprContext(ExprContext *econtext, bool isCommit);
 extern void ReScanExprContext(ExprContext *econtext);