Reduce memory usage of targetlist SRFs.

Previously nodeProjectSet only released memory once per input tuple,
rather than once per returned tuple. If the computation of an
individual returned tuple requires a lot of memory, that can lead to
problems.

Instead change things so that the expression context can be reset once
per output tuple, which requires a new memory context to store SRF
arguments in.

This is a longstanding issue, but was hard to fix before 9.6, due to
the way tSRFs where evaluated. But it's fairly easy to fix now. We
could backpatch this into 10, but given there've been fewc omplaints
that doesn't seem worth the risk so far.

Reported-By: Lucas Fairchild
Author: Andres Freund, per discussion with Tom Lane
Discussion: https://postgr.es/m/4514.1507318623@sss.pgh.pa.us

src/backend/executor/execSRF.c

@@ -467,13 +467,16 @@ ExecInitFunctionResultSet(Expr *expr,
  * function itself.  The argument expressions may not contain set-returning
  * functions (the planner is supposed to have separated evaluation for those).
  *
- * This should be called in a short-lived (per-tuple) context.
+ * This should be called in a short-lived (per-tuple) context, argContext
+ * needs to live until all rows have been returned (i.e. *isDone set to
+ * ExprEndResult or ExprSingleResult).
  *
  * This is used by nodeProjectSet.c.
  */
 Datum
 ExecMakeFunctionResultSet(SetExprState *fcache,
 						  ExprContext *econtext,
+						  MemoryContext argContext,
 						  bool *isNull,
 						  ExprDoneCond *isDone)
 {
@@ -497,8 +500,21 @@ restart:
 	 */
 	if (fcache->funcResultStore)
 	{
-		if (tuplestore_gettupleslot(fcache->funcResultStore, true, false,
-									fcache->funcResultSlot))
+		TupleTableSlot *slot = fcache->funcResultSlot;
+		MemoryContext oldContext;
+		bool foundTup;
+
+		/*
+		 * Have to make sure tuple in slot lives long enough, otherwise
+		 * clearing the slot could end up trying to free something already
+		 * freed.
+		 */
+		oldContext = MemoryContextSwitchTo(slot->tts_mcxt);
+		foundTup = tuplestore_gettupleslot(fcache->funcResultStore, true, false,
+										   fcache->funcResultSlot);
+		MemoryContextSwitchTo(oldContext);
+
+		if (foundTup)
 		{
 			*isDone = ExprMultipleResult;
 			if (fcache->funcReturnsTuple)
@@ -526,11 +542,20 @@ restart:
 	 * function manager.  We skip the evaluation if it was already done in the
 	 * previous call (ie, we are continuing the evaluation of a set-valued
 	 * function).  Otherwise, collect the current argument values into fcinfo.
+	 *
+	 * The arguments have to live in a context that lives at least until all
+	 * rows from this SRF have been returned, otherwise ValuePerCall SRFs
+	 * would reference freed memory after the first returned row.
 	 */
 	fcinfo = &fcache->fcinfo_data;
 	arguments = fcache->args;
 	if (!fcache->setArgsValid)
+	{
+		MemoryContext oldContext = MemoryContextSwitchTo(argContext);
+
 		ExecEvalFuncArgs(fcinfo, arguments, econtext);
+		MemoryContextSwitchTo(oldContext);
+	}
 	else
 	{
 		/* Reset flag (we may set it again below) */

src/backend/executor/nodeProjectSet.c

@@ -52,6 +52,13 @@ ExecProjectSet(PlanState *pstate)
 
 	econtext = node->ps.ps_ExprContext;
 
+	/*
+	 * Reset per-tuple context to free expression-evaluation storage allocated
+	 * for a potentially previously returned tuple. Note that the SRF argument
+	 * context has a different lifetime and is reset below.
+	 */
+	ResetExprContext(econtext);
+
 	/*
 	 * Check to see if we're still projecting out tuples from a previous scan
 	 * tuple (because there is a function-returning-set in the projection
@@ -66,11 +73,13 @@ ExecProjectSet(PlanState *pstate)
 	}
 
 	/*
-	 * Reset per-tuple memory context to free any expression evaluation
-	 * storage allocated in the previous tuple cycle.  Note this can't happen
-	 * until we're done projecting out tuples from a scan tuple.
+	 * Reset argument context to free any expression evaluation storage
+	 * allocated in the previous tuple cycle.  Note this can't happen until
+	 * we're done projecting out tuples from a scan tuple, as ValuePerCall
+	 * functions are allowed to reference the arguments for each returned
+	 * tuple.
 	 */
-	ResetExprContext(econtext);
+	MemoryContextReset(node->argcontext);
 
 	/*
 	 * Get another input tuple and project SRFs from it.
@@ -164,7 +173,8 @@ ExecProjectSRF(ProjectSetState *node, bool continuing)
 			 * Evaluate SRF - possibly continuing previously started output.
 			 */
 			*result = ExecMakeFunctionResultSet((SetExprState *) elem,
-												econtext, isnull, isdone);
+												econtext, node->argcontext,
+												isnull, isdone);
 
 			if (*isdone != ExprEndResult)
 				hasresult = true;
@@ -291,6 +301,18 @@ ExecInitProjectSet(ProjectSet *node, EState *estate, int eflags)
 		off++;
 	}
 
+
+	/*
+	 * Create a memory context that ExecMakeFunctionResult can use to evaluate
+	 * function arguments in.  We can't use the per-tuple context for this
+	 * because it gets reset too often; but we don't want to leak evaluation
+	 * results into the query-lifespan context either.  We use one context for
+	 * the arguments of all tSRFs, as they have roughly equivalent lifetimes.
+	 */
+	state->argcontext = AllocSetContextCreate(CurrentMemoryContext,
+											  "tSRF function arguments",
+											  ALLOCSET_DEFAULT_SIZES);
+
 	return state;
 }
 

src/include/executor/executor.h

@@ -400,6 +400,7 @@ extern SetExprState *ExecInitFunctionResultSet(Expr *expr,
 						  ExprContext *econtext, PlanState *parent);
 extern Datum ExecMakeFunctionResultSet(SetExprState *fcache,
 						  ExprContext *econtext,
+						  MemoryContext argContext,
 						  bool *isNull,
 						  ExprDoneCond *isDone);
 

src/include/nodes/execnodes.h

@@ -946,6 +946,7 @@ typedef struct ProjectSetState
 	ExprDoneCond *elemdone;		/* array of per-SRF is-done states */
 	int			nelems;			/* length of elemdone[] array */
 	bool		pending_srf_tuples; /* still evaluating srfs in tlist? */
+	MemoryContext argcontext;	/* context for SRF arguments */
 } ProjectSetState;
 
 /* ----------------