TABLESAMPLE, SQL Standard and extensible

Add a TABLESAMPLE clause to SELECT statements that allows
user to specify random BERNOULLI sampling or block level
SYSTEM sampling. Implementation allows for extensible
sampling functions to be written, using a standard API.
Basic version follows SQLStandard exactly. Usable
concrete use cases for the sampling API follow in later
commits.

Petr Jelinek

Reviewed by Michael Paquier and Simon Riggs

1 files changed, 22 insertions, 0 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index faca30b322d..ea7a47bdf45 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -706,6 +706,26 @@ create_seqscan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer)
 }
 
 /*
+ * create_samplescan_path
+ *	  Like seqscan but uses sampling function while scanning.
+ */
+Path *
+create_samplescan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer)
+{
+	Path		   *pathnode = makeNode(Path);
+
+	pathnode->pathtype = T_SampleScan;
+	pathnode->parent = rel;
+	pathnode->param_info = get_baserel_parampathinfo(root, rel,
+													 required_outer);
+	pathnode->pathkeys = NIL;	/* samplescan has unordered result */
+
+	cost_samplescan(pathnode, root, rel);
+
+	return pathnode;
+}
+
+/*
  * create_index_path
  *	  Creates a path node for an index scan.
  *
@@ -1778,6 +1798,8 @@ reparameterize_path(PlannerInfo *root, Path *path,
 		case T_SubqueryScan:
 			return create_subqueryscan_path(root, rel, path->pathkeys,
 											required_outer);
+		case T_SampleScan:
+			return (Path *) create_samplescan_path(root, rel, required_outer);
 		default:
 			break;
 	}