Refactor create_limit_path() to share cost adjustment code with FDWs.

This is in preparation for an upcoming commit.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska and Jeff Janes
Discussion: https://postgr.es/m/87pnz1aby9.fsf@news-spur.riddles.org.uk

src/backend/optimizer/util/pathnode.c

@@ -3578,17 +3578,42 @@ create_limit_path(PlannerInfo *root, RelOptInfo *rel,
 
 	/*
 	 * Adjust the output rows count and costs according to the offset/limit.
-	 * This is only a cosmetic issue if we are at top level, but if we are
-	 * building a subquery then it's important to report correct info to the
-	 * outer planner.
-	 *
-	 * When the offset or count couldn't be estimated, use 10% of the
-	 * estimated number of rows emitted from the subpath.
-	 *
-	 * XXX we don't bother to add eval costs of the offset/limit expressions
-	 * themselves to the path costs.  In theory we should, but in most cases
-	 * those expressions are trivial and it's just not worth the trouble.
 	 */
+	adjust_limit_rows_costs(&pathnode->path.rows,
+							&pathnode->path.startup_cost,
+							&pathnode->path.total_cost,
+							offset_est, count_est);
+
+	return pathnode;
+}
+
+/*
+ * adjust_limit_rows_costs
+ *	  Adjust the size and cost estimates for a LimitPath node according to the
+ *	  offset/limit.
+ *
+ * This is only a cosmetic issue if we are at top level, but if we are
+ * building a subquery then it's important to report correct info to the outer
+ * planner.
+ *
+ * When the offset or count couldn't be estimated, use 10% of the estimated
+ * number of rows emitted from the subpath.
+ *
+ * XXX we don't bother to add eval costs of the offset/limit expressions
+ * themselves to the path costs.  In theory we should, but in most cases those
+ * expressions are trivial and it's just not worth the trouble.
+ */
+void
+adjust_limit_rows_costs(double *rows,	/* in/out parameter */
+						Cost *startup_cost,	/* in/out parameter */
+						Cost *total_cost,	/* in/out parameter */
+						int64 offset_est,
+						int64 count_est)
+{
+	double		input_rows = *rows;
+	Cost		input_startup_cost = *startup_cost;
+	Cost		input_total_cost = *total_cost;
+
 	if (offset_est != 0)
 	{
 		double		offset_rows;
@@ -3596,16 +3621,16 @@ create_limit_path(PlannerInfo *root, RelOptInfo *rel,
 		if (offset_est > 0)
 			offset_rows = (double) offset_est;
 		else
-			offset_rows = clamp_row_est(subpath->rows * 0.10);
-		if (offset_rows > pathnode->path.rows)
-			offset_rows = pathnode->path.rows;
-		if (subpath->rows > 0)
-			pathnode->path.startup_cost +=
-				(subpath->total_cost - subpath->startup_cost)
-				* offset_rows / subpath->rows;
-		pathnode->path.rows -= offset_rows;
-		if (pathnode->path.rows < 1)
-			pathnode->path.rows = 1;
+			offset_rows = clamp_row_est(input_rows * 0.10);
+		if (offset_rows > *rows)
+			offset_rows = *rows;
+		if (input_rows > 0)
+			*startup_cost +=
+				(input_total_cost - input_startup_cost)
+				* offset_rows / input_rows;
+		*rows -= offset_rows;
+		if (*rows < 1)
+			*rows = 1;
 	}
 
 	if (count_est != 0)
@@ -3615,19 +3640,17 @@ create_limit_path(PlannerInfo *root, RelOptInfo *rel,
 		if (count_est > 0)
 			count_rows = (double) count_est;
 		else
-			count_rows = clamp_row_est(subpath->rows * 0.10);
-		if (count_rows > pathnode->path.rows)
-			count_rows = pathnode->path.rows;
-		if (subpath->rows > 0)
-			pathnode->path.total_cost = pathnode->path.startup_cost +
-				(subpath->total_cost - subpath->startup_cost)
-				* count_rows / subpath->rows;
-		pathnode->path.rows = count_rows;
-		if (pathnode->path.rows < 1)
-			pathnode->path.rows = 1;
+			count_rows = clamp_row_est(input_rows * 0.10);
+		if (count_rows > *rows)
+			count_rows = *rows;
+		if (input_rows > 0)
+			*total_cost = *startup_cost +
+				(input_total_cost - input_startup_cost)
+				* count_rows / input_rows;
+		*rows = count_rows;
+		if (*rows < 1)
+			*rows = 1;
 	}
-
-	return pathnode;
 }
 
 

src/include/optimizer/pathnode.h

@@ -265,6 +265,9 @@ extern LimitPath *create_limit_path(PlannerInfo *root, RelOptInfo *rel,
 				  Path *subpath,
 				  Node *limitOffset, Node *limitCount,
 				  int64 offset_est, int64 count_est);
+extern void adjust_limit_rows_costs(double *rows,
+						Cost *startup_cost, Cost *total_cost,
+						int64 offset_est, int64 count_est);
 
 extern Path *reparameterize_path(PlannerInfo *root, Path *path,
 					Relids required_outer,