Assign each subquery a unique name prior to planning it.

Previously, subqueries were given names only after they were planned,
which makes it difficult to use information from a previous execution of
the query to guide future planning. If, for example, you knew something
about how you want "InitPlan 2" to be planned, you won't know whether
the subquery you're currently planning will end up being "InitPlan 2"
until after you've finished planning it, by which point it's too late to
use the information that you had.

To fix this, assign each subplan a unique name before we begin planning
it. To improve consistency, use textual names for all subplans, rather
than, as we did previously, a mix of numbers (such as "InitPlan 1") and
names (such as "CTE foo"), and make sure that the same name is never
assigned more than once.

We adopt the somewhat arbitrary convention of using the type of sublink
to set the plan name; for example, a query that previously had two
expression sublinks shown as InitPlan 2 and InitPlan 1 will now end up
named expr_1 and expr_2. Because names are assigned before rather than
after planning, some of the regression test outputs show the numerical
part of the name switching positions: what was previously SubPlan 2 was
actually the first one encountered, but we finished planning it later.

We assign names even to subqueries that aren't shown as such within the
EXPLAIN output. These include subqueries that are a FROM clause item or
a branch of a set operation, rather than something that will be turned
into an InitPlan or SubPlan. The purpose of this is to make sure that,
below the topmost query level, there's always a name for each subquery
that is stable from one planning cycle to the next (assuming no changes
to the query or the database schema).

Author: Robert Haas <rhaas@postgresql.org>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
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>
Discussion: http://postgr.es/m/3641043.1758751399@sss.pgh.pa.us

1 files changed, 74 insertions, 4 deletions

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 41bd8353430..3b130e724f7 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -439,7 +439,8 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	}
 
 	/* primary planning entry point (may recurse for subqueries) */
-	root = subquery_planner(glob, parse, NULL, false, tuple_fraction, NULL);
+	root = subquery_planner(glob, parse, NULL, NULL, false, tuple_fraction,
+							NULL);
 
 	/* Select best Path and turn it into a Plan */
 	final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL);
@@ -630,6 +631,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
  *
  * glob is the global state for the current planner run.
  * parse is the querytree produced by the parser & rewriter.
+ * plan_name is the name to assign to this subplan (NULL at the top level).
  * parent_root is the immediate parent Query's info (NULL at the top level).
  * hasRecursion is true if this is a recursive WITH query.
  * tuple_fraction is the fraction of tuples we expect will be retrieved.
@@ -656,9 +658,9 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
  *--------------------
  */
 PlannerInfo *
-subquery_planner(PlannerGlobal *glob, Query *parse, PlannerInfo *parent_root,
-				 bool hasRecursion, double tuple_fraction,
-				 SetOperationStmt *setops)
+subquery_planner(PlannerGlobal *glob, Query *parse, char *plan_name,
+				 PlannerInfo *parent_root, bool hasRecursion,
+				 double tuple_fraction, SetOperationStmt *setops)
 {
 	PlannerInfo *root;
 	List	   *newWithCheckOptions;
@@ -673,6 +675,7 @@ subquery_planner(PlannerGlobal *glob, Query *parse, PlannerInfo *parent_root,
 	root->parse = parse;
 	root->glob = glob;
 	root->query_level = parent_root ? parent_root->query_level + 1 : 1;
+	root->plan_name = plan_name;
 	root->parent_root = parent_root;
 	root->plan_params = NIL;
 	root->outer_params = NULL;
@@ -8833,3 +8836,70 @@ create_partial_unique_paths(PlannerInfo *root, RelOptInfo *input_rel,
 								  sjinfo, unique_rel);
 	}
 }
+
+/*
+ * Choose a unique name for some subroot.
+ *
+ * Modifies glob->subplanNames to track names already used.
+ */
+char *
+choose_plan_name(PlannerGlobal *glob, const char *name, bool always_number)
+{
+	unsigned	n;
+
+	/*
+	 * If a numeric suffix is not required, then search the list of
+	 * previously-assigned names for a match. If none is found, then we can
+	 * use the provided name without modification.
+	 */
+	if (!always_number)
+	{
+		bool		found = false;
+
+		foreach_ptr(char, subplan_name, glob->subplanNames)
+		{
+			if (strcmp(subplan_name, name) == 0)
+			{
+				found = true;
+				break;
+			}
+		}
+
+		if (!found)
+		{
+			/* pstrdup here is just to avoid cast-away-const */
+			char	   *chosen_name = pstrdup(name);
+
+			glob->subplanNames = lappend(glob->subplanNames, chosen_name);
+			return chosen_name;
+		}
+	}
+
+	/*
+	 * If a numeric suffix is required or if the un-suffixed name is already
+	 * in use, then loop until we find a positive integer that produces a
+	 * novel name.
+	 */
+	for (n = 1; true; ++n)
+	{
+		char	   *proposed_name = psprintf("%s_%u", name, n);
+		bool		found = false;
+
+		foreach_ptr(char, subplan_name, glob->subplanNames)
+		{
+			if (strcmp(subplan_name, proposed_name) == 0)
+			{
+				found = true;
+				break;
+			}
+		}
+
+		if (!found)
+		{
+			glob->subplanNames = lappend(glob->subplanNames, proposed_name);
+			return proposed_name;
+		}
+
+		pfree(proposed_name);
+	}
+}