Don't generate EEOP_*_FETCHSOME operations for slots know to be virtual.

That avoids unnecessary work during both interpreted execution, and
JIT compiled expression evaluation. Both benefit from fewer expression
steps needing be processed, and for interpreted execution there now is
a fastpath dedicated to just fetching a value from a virtual
slot. That's e.g. beneficial for hashjoins over nodes that perform
projections, as the hashed columns are currently fetched individually.

Author: Soumyadeep Chakraborty, Andres Freund
Discussion: https://postgr.es/m/CAE-ML+9OKSN71+mHtfMD-L24oDp8dGTfaVjDU6U+j+FNAW5kRQ@mail.gmail.com

src/backend/executor/execExpr.c

@@ -65,7 +65,7 @@ static void ExecInitFunc(ExprEvalStep *scratch, Expr *node, List *args,
 static void ExecInitExprSlots(ExprState *state, Node *node);
 static void ExecPushExprSlots(ExprState *state, LastAttnumInfo *info);
 static bool get_last_attnums_walker(Node *node, LastAttnumInfo *info);
-static void ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op);
+static bool ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op);
 static void ExecInitWholeRowVar(ExprEvalStep *scratch, Var *variable,
 								ExprState *state);
 static void ExecInitSubscriptingRef(ExprEvalStep *scratch,
@@ -2285,8 +2285,8 @@ ExecPushExprSlots(ExprState *state, LastAttnumInfo *info)
 		scratch.d.fetch.fixed = false;
 		scratch.d.fetch.kind = NULL;
 		scratch.d.fetch.known_desc = NULL;
-		ExecComputeSlotInfo(state, &scratch);
+		if (ExecComputeSlotInfo(state, &scratch))
-		ExprEvalPushStep(state, &scratch);
+			ExprEvalPushStep(state, &scratch);
 	}
 	if (info->last_outer > 0)
 	{
@@ -2295,8 +2295,8 @@ ExecPushExprSlots(ExprState *state, LastAttnumInfo *info)
 		scratch.d.fetch.fixed = false;
 		scratch.d.fetch.kind = NULL;
 		scratch.d.fetch.known_desc = NULL;
-		ExecComputeSlotInfo(state, &scratch);
+		if (ExecComputeSlotInfo(state, &scratch))
-		ExprEvalPushStep(state, &scratch);
+			ExprEvalPushStep(state, &scratch);
 	}
 	if (info->last_scan > 0)
 	{
@@ -2305,8 +2305,8 @@ ExecPushExprSlots(ExprState *state, LastAttnumInfo *info)
 		scratch.d.fetch.fixed = false;
 		scratch.d.fetch.kind = NULL;
 		scratch.d.fetch.known_desc = NULL;
-		ExecComputeSlotInfo(state, &scratch);
+		if (ExecComputeSlotInfo(state, &scratch))
-		ExprEvalPushStep(state, &scratch);
+			ExprEvalPushStep(state, &scratch);
 	}
 }
 
@@ -2364,14 +2364,21 @@ get_last_attnums_walker(Node *node, LastAttnumInfo *info)
  * The goal is to determine whether a slot is 'fixed', that is, every
  * evaluation of the expression will have the same type of slot, with an
  * equivalent descriptor.
+ *
+ * Returns true if the the deforming step is required, false otherwise.
  */
-static void
+static bool
 ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 {
 	PlanState  *parent = state->parent;
 	TupleDesc	desc = NULL;
 	const TupleTableSlotOps *tts_ops = NULL;
 	bool		isfixed = false;
+	ExprEvalOp	opcode = op->opcode;
+
+	Assert(opcode == EEOP_INNER_FETCHSOME ||
+		   opcode == EEOP_OUTER_FETCHSOME ||
+		   opcode == EEOP_SCAN_FETCHSOME);
 
 	if (op->d.fetch.known_desc != NULL)
 	{
@@ -2383,7 +2390,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 	{
 		isfixed = false;
 	}
-	else if (op->opcode == EEOP_INNER_FETCHSOME)
+	else if (opcode == EEOP_INNER_FETCHSOME)
 	{
 		PlanState  *is = innerPlanState(parent);
 
@@ -2403,7 +2410,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 			desc = ExecGetResultType(is);
 		}
 	}
-	else if (op->opcode == EEOP_OUTER_FETCHSOME)
+	else if (opcode == EEOP_OUTER_FETCHSOME)
 	{
 		PlanState  *os = outerPlanState(parent);
 
@@ -2423,7 +2430,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 			desc = ExecGetResultType(os);
 		}
 	}
-	else if (op->opcode == EEOP_SCAN_FETCHSOME)
+	else if (opcode == EEOP_SCAN_FETCHSOME)
 	{
 		desc = parent->scandesc;
 
@@ -2446,6 +2453,12 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 		op->d.fetch.kind = NULL;
 		op->d.fetch.known_desc = NULL;
 	}
+
+	/* if the slot is known to always virtual we never need to deform */
+	if (op->d.fetch.fixed && op->d.fetch.kind == &TTSOpsVirtual)
+		return false;
+
+	return true;
 }
 
 /*
@@ -3360,16 +3373,16 @@ ExecBuildGroupingEqual(TupleDesc ldesc, TupleDesc rdesc,
 	scratch.d.fetch.fixed = false;
 	scratch.d.fetch.known_desc = ldesc;
 	scratch.d.fetch.kind = lops;
-	ExecComputeSlotInfo(state, &scratch);
+	if (ExecComputeSlotInfo(state, &scratch))
-	ExprEvalPushStep(state, &scratch);
+		ExprEvalPushStep(state, &scratch);
 
 	scratch.opcode = EEOP_OUTER_FETCHSOME;
 	scratch.d.fetch.last_var = maxatt;
 	scratch.d.fetch.fixed = false;
 	scratch.d.fetch.known_desc = rdesc;
 	scratch.d.fetch.kind = rops;
-	ExecComputeSlotInfo(state, &scratch);
+	if (ExecComputeSlotInfo(state, &scratch))
-	ExprEvalPushStep(state, &scratch);
+		ExprEvalPushStep(state, &scratch);
 
 	/*
 	 * Start comparing at the last field (least significant sort key). That's

src/backend/executor/execExprInterp.c

@@ -160,6 +160,12 @@ static Datum ExecJustAssignOuterVar(ExprState *state, ExprContext *econtext, boo
 static Datum ExecJustAssignScanVar(ExprState *state, ExprContext *econtext, bool *isnull);
 static Datum ExecJustApplyFuncToCase(ExprState *state, ExprContext *econtext, bool *isnull);
 static Datum ExecJustConst(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustInnerVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustOuterVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustScanVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustAssignInnerVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustAssignOuterVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
+static Datum ExecJustAssignScanVarVirt(ExprState *state, ExprContext *econtext, bool *isnull);
 
 
 /*
@@ -255,11 +261,45 @@ ExecReadyInterpretedExpr(ExprState *state)
 			return;
 		}
 	}
-	else if (state->steps_len == 2 &&
+	else if (state->steps_len == 2)
-			 state->steps[0].opcode == EEOP_CONST)
 	{
-		state->evalfunc_private = (void *) ExecJustConst;
+		ExprEvalOp	step0 = state->steps[0].opcode;
-		return;
+
+		if (step0 == EEOP_CONST)
+		{
+			state->evalfunc_private = (void *) ExecJustConst;
+			return;
+		}
+		else if (step0 == EEOP_INNER_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustInnerVarVirt;
+			return;
+		}
+		else if (step0 == EEOP_OUTER_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustOuterVarVirt;
+			return;
+		}
+		else if (step0 == EEOP_SCAN_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustScanVarVirt;
+			return;
+		}
+		else if (step0 == EEOP_ASSIGN_INNER_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustAssignInnerVarVirt;
+			return;
+		}
+		else if (step0 == EEOP_ASSIGN_OUTER_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustAssignOuterVarVirt;
+			return;
+		}
+		else if (step0 == EEOP_ASSIGN_SCAN_VAR)
+		{
+			state->evalfunc_private = (void *) ExecJustAssignScanVarVirt;
+			return;
+		}
 	}
 
 #if defined(EEO_USE_COMPUTED_GOTO)
@@ -2096,6 +2136,91 @@ ExecJustConst(ExprState *state, ExprContext *econtext, bool *isnull)
 	return op->d.constval.value;
 }
 
+/* implementation of ExecJust(Inner|Outer|Scan)VarVirt */
+static pg_attribute_always_inline Datum
+ExecJustVarVirtImpl(ExprState *state, TupleTableSlot *slot, bool *isnull)
+{
+	ExprEvalStep *op = &state->steps[0];
+	int			attnum = op->d.var.attnum;
+
+	/*
+	 * As it is guaranteed that a virtual slot is used, there never is a need
+	 * to perform tuple deforming (nor would it be possible). Therefore
+	 * execExpr.c has not emitted an EEOP_*_FETCHSOME step. Verify, as much as
+	 * possible, that that determination was accurate.
+	 */
+	Assert(TTS_IS_VIRTUAL(slot));
+	Assert(TTS_FIXED(slot));
+	Assert(attnum >= 0 && attnum < slot->tts_nvalid);
+
+	*isnull = slot->tts_isnull[attnum];
+
+	return slot->tts_values[attnum];
+}
+
+/* Like ExecJustInnerVar, optimized for virtual slots */
+static Datum
+ExecJustInnerVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustVarVirtImpl(state, econtext->ecxt_innertuple, isnull);
+}
+
+/* Like ExecJustOuterVar, optimized for virtual slots */
+static Datum
+ExecJustOuterVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustVarVirtImpl(state, econtext->ecxt_outertuple, isnull);
+}
+
+/* Like ExecJustScanVar, optimized for virtual slots */
+static Datum
+ExecJustScanVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustVarVirtImpl(state, econtext->ecxt_scantuple, isnull);
+}
+
+/* implementation of ExecJustAssign(Inner|Outer|Scan)VarVirt */
+static pg_attribute_always_inline Datum
+ExecJustAssignVarVirtImpl(ExprState *state, TupleTableSlot *inslot, bool *isnull)
+{
+	ExprEvalStep *op = &state->steps[0];
+	int			attnum = op->d.assign_var.attnum;
+	int			resultnum = op->d.assign_var.resultnum;
+	TupleTableSlot *outslot = state->resultslot;
+
+	/* see ExecJustVarVirtImpl for comments */
+
+	Assert(TTS_IS_VIRTUAL(inslot));
+	Assert(TTS_FIXED(inslot));
+	Assert(attnum >= 0 && attnum < inslot->tts_nvalid);
+
+	outslot->tts_values[resultnum] = inslot->tts_values[attnum];
+	outslot->tts_isnull[resultnum] = inslot->tts_isnull[attnum];
+
+	return 0;
+}
+
+/* Like ExecJustAssignInnerVar, optimized for virtual slots */
+static Datum
+ExecJustAssignInnerVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustAssignVarVirtImpl(state, econtext->ecxt_innertuple, isnull);
+}
+
+/* Like ExecJustAssignOuterVar, optimized for virtual slots */
+static Datum
+ExecJustAssignOuterVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustAssignVarVirtImpl(state, econtext->ecxt_outertuple, isnull);
+}
+
+/* Like ExecJustAssignScanVar, optimized for virtual slots */
+static Datum
+ExecJustAssignScanVarVirt(ExprState *state, ExprContext *econtext, bool *isnull)
+{
+	return ExecJustAssignVarVirtImpl(state, econtext->ecxt_scantuple, isnull);
+}
+
 #if defined(EEO_USE_COMPUTED_GOTO)
 /*
  * Comparator used when building address->opcode lookup table for

src/backend/jit/llvm/llvmjit_expr.c

@@ -287,6 +287,9 @@ llvm_compile_expr(ExprState *state)
 					if (op->d.fetch.fixed)
 						tts_ops = op->d.fetch.kind;
 
+					/* step should not have been generated */
+					Assert(tts_ops != &TTSOpsVirtual);
+
 					if (opcode == EEOP_INNER_FETCHSOME)
 						v_slot = v_innerslot;
 					else if (opcode == EEOP_OUTER_FETCHSOME)
@@ -297,9 +300,6 @@ llvm_compile_expr(ExprState *state)
 					/*
 					 * Check if all required attributes are available, or
 					 * whether deforming is required.
-					 *
-					 * TODO: skip nvalid check if slot is fixed and known to
-					 * be a virtual slot.
 					 */
 					v_nvalid =
 						l_load_struct_gep(b, v_slot,