Clean up handling of constraint_exclusion and enable_partition_pruning.

The interaction of these parameters was a bit confused/confusing,
and in fact v11 entirely misses the opportunity to apply partition
constraints when a partition is accessed directly (rather than
indirectly from its parent).

In HEAD, establish the principle that enable_partition_pruning controls
partition pruning and nothing else.  When accessing a partition via its
parent, we do partition pruning (if enabled by enable_partition_pruning)
and then there is no need to consider partition constraints in the
constraint_exclusion logic.  When accessing a partition directly, its
partition constraints are applied by the constraint_exclusion logic,
only if constraint_exclusion = on.

In v11, we can't have such a clean division of these GUCs' effects,
partly because we don't want to break compatibility too much in a
released branch, and partly because the clean coding requires
inheritance_planner to have applied partition pruning to a partitioned
target table, which it doesn't in v11.  However, we can tweak things
enough to cover the missed case, which seems like a good idea since
it's potentially a performance regression from v10.  This patch keeps
v11's previous behavior in which enable_partition_pruning overrides
constraint_exclusion for an inherited target table, though.

In HEAD, also teach relation_excluded_by_constraints that it's okay to use
inheritable constraints when trying to prune a traditional inheritance
tree.  This might not be thought worthy of effort given that that feature
is semi-deprecated now, but we have enough infrastructure that it only
takes a couple more lines of code to do it correctly.

Amit Langote and Tom Lane

Discussion: https://postgr.es/m/[email protected]
Discussion: https://postgr.es/m/[email protected]

2 files changed, 62 insertions, 0 deletions

diff --git a/src/test/regress/expected/partition_prune.out b/src/test/regress/expected/partition_prune.out
index 0789b316eb4..bd64bed8fc7 100644
--- a/src/test/regress/expected/partition_prune.out
+++ b/src/test/regress/expected/partition_prune.out
@@ -3639,4 +3639,46 @@ select * from listp where a = (select 2) and b <> 10;
      ->  Result (never executed)
 (4 rows)
 
+--
+-- check that a partition directly accessed in a query is excluded with
+-- constraint_exclusion = on
+--
+-- turn off partition pruning, so that it doesn't interfere
+set enable_partition_pruning to off;
+-- setting constraint_exclusion to 'partition' disables exclusion
+set constraint_exclusion to 'partition';
+explain (costs off) select * from listp1 where a = 2;
+     QUERY PLAN     
+--------------------
+ Seq Scan on listp1
+   Filter: (a = 2)
+(2 rows)
+
+explain (costs off) update listp1 set a = 1 where a = 2;
+        QUERY PLAN        
+--------------------------
+ Update on listp1
+   ->  Seq Scan on listp1
+         Filter: (a = 2)
+(3 rows)
+
+-- constraint exclusion enabled
+set constraint_exclusion to 'on';
+explain (costs off) select * from listp1 where a = 2;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+explain (costs off) update listp1 set a = 1 where a = 2;
+           QUERY PLAN           
+--------------------------------
+ Update on listp1
+   ->  Result
+         One-Time Filter: false
+(3 rows)
+
+reset constraint_exclusion;
+reset enable_partition_pruning;
 drop table listp;
diff --git a/src/test/regress/sql/partition_prune.sql b/src/test/regress/sql/partition_prune.sql
index c30e58eef73..246c6274af6 100644
--- a/src/test/regress/sql/partition_prune.sql
+++ b/src/test/regress/sql/partition_prune.sql
@@ -990,4 +990,24 @@ create table listp2_10 partition of listp2 for values in (10);
 explain (analyze, costs off, summary off, timing off)
 select * from listp where a = (select 2) and b <> 10;
 
+--
+-- check that a partition directly accessed in a query is excluded with
+-- constraint_exclusion = on
+--
+
+-- turn off partition pruning, so that it doesn't interfere
+set enable_partition_pruning to off;
+
+-- setting constraint_exclusion to 'partition' disables exclusion
+set constraint_exclusion to 'partition';
+explain (costs off) select * from listp1 where a = 2;
+explain (costs off) update listp1 set a = 1 where a = 2;
+-- constraint exclusion enabled
+set constraint_exclusion to 'on';
+explain (costs off) select * from listp1 where a = 2;
+explain (costs off) update listp1 set a = 1 where a = 2;
+
+reset constraint_exclusion;
+reset enable_partition_pruning;
+
 drop table listp;