Keep track of what RTIs a Result node is scanning.

Result nodes now include an RTI set, which is only non-NULL when they
have no subplan, and is taken from the relid set of the RelOptInfo that
the Result is generating. ExplainPreScanNode now takes notice of these
RTIs, which means that a few things get schema-qualified in the
regression tests that previously did not. This makes the output more
consistent between cases where some part of the plan tree is replaced by
a Result node and those where this does not happen.

Likewise, pg_overexplain's EXPLAIN (RANGE_TABLE) now displays the RTIs
stored in a Result node just as it already does for other RTI-bearing
node types.

Result nodes also now include a result_reason, which tells us something
about why the Result node was inserted.  Using that information, EXPLAIN
now emits, where relevant, a "Replaces" line describing the origin of
a Result node.

The purpose of these changes is to allow code that inspects a Plan
tree to understand the origin of Result nodes that appear therein.

Discussion: http://postgr.es/m/CA+TgmoYeUZePZWLsSO+1FAN7UPePT_RMEZBKkqYBJVCF1s60=w@mail.gmail.com
Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>

2 files changed, 16 insertions, 2 deletions

diff --git a/contrib/pg_overexplain/expected/pg_overexplain.out b/contrib/pg_overexplain/expected/pg_overexplain.out
index 6de02323d7c..55d34666d87 100644
--- a/contrib/pg_overexplain/expected/pg_overexplain.out
+++ b/contrib/pg_overexplain/expected/pg_overexplain.out
@@ -44,9 +44,10 @@ EXPLAIN (RANGE_TABLE) SELECT 1;
                 QUERY PLAN                
 ------------------------------------------
  Result  (cost=0.00..0.01 rows=1 width=4)
+   RTIs: 1
  RTI 1 (result):
    Eref: "*RESULT*" ()
-(3 rows)
+(4 rows)
 
 -- Create a partitioned table.
 CREATE TABLE vegetables (id serial, name text, genus text)
@@ -475,6 +476,7 @@ INSERT INTO vegetables (name, genus) VALUES ('broccoflower', 'brassica');
    Nominal RTI: 1
    Exclude Relation RTI: 0
    ->  Result
+         RTIs: 2
  RTI 1 (relation):
    Eref: vegetables (id, name, genus)
    Relation: vegetables
@@ -485,5 +487,5 @@ INSERT INTO vegetables (name, genus) VALUES ('broccoflower', 'brassica');
    Eref: "*RESULT*" ()
  Unprunable RTIs: 1
  Result RTIs: 1
-(14 rows)
+(15 rows)
 
diff --git a/contrib/pg_overexplain/pg_overexplain.c b/contrib/pg_overexplain/pg_overexplain.c
index de824566f8c..bd70b6d9d5e 100644
--- a/contrib/pg_overexplain/pg_overexplain.c
+++ b/contrib/pg_overexplain/pg_overexplain.c
@@ -236,6 +236,18 @@ overexplain_per_node_hook(PlanState *planstate, List *ancestors,
 									  ((MergeAppend *) plan)->apprelids,
 									  es);
 				break;
+			case T_Result:
+
+				/*
+				 * 'relids' is only meaningful when plan->lefttree is NULL,
+				 * but if somehow it ends up set when plan->lefttree is not
+				 * NULL, print it anyway.
+				 */
+				if (plan->lefttree == NULL ||
+					((Result *) plan)->relids != NULL)
+					overexplain_bitmapset("RTIs",
+										  ((Result *) plan)->relids,
+										  es);
 			default:
 				break;
 		}