When I run a query like below on a child-less table, the plan comes out to be
explain verbose SELECT * FROM uprt1_l WHERE a = 1 AND a = 2;
QUERY PLAN
----------------------------------------------------------------------
Result (cost=0.00..11.50 rows=1 width=13)
Output: a, b, c
One-Time Filter: false
-> Seq Scan on public.uprt1_l (cost=0.00..11.50 rows=1 width=13)
Output: a, b, c
Filter: (uprt1_l.a = 1)
(6 rows)
where as the same query run on a parent with children, the plan is
postgres=# \d prt1_l
Table "public.prt1_l"
Column | Type | Collation | Nullable | Default
--------+-------------------+-----------+----------+---------
a | integer | | not null |
b | integer | | |
c | character varying | | |
Partition key: RANGE (a)
Number of partitions: 3 (Use \d+ to list them.)
postgres=# explain verbose SELECT * FROM prt1_l WHERE a = 1 AND a = 2;
QUERY PLAN
-------------------------------------------
Result (cost=0.00..0.00 rows=0 width=40)
Output: prt1_l.a, prt1_l.b, prt1_l.c
One-Time Filter: false
(3 rows)
For a parent table with children, set_append_rel_size() evaluates
restrictions in loop
880 foreach(l, root->append_rel_list)
881 {
882 AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l);
starting at 1021. If any of the restrictions are evaluated to false,
it set the child as dummy. If all children are dummy, the appendrel is
set to dummy.
But for a child-less table, even if the "false" qual is available in
baserestrictinfo in set_rel_size(), we do not mark the relation as
dummy. Instead, paths are created for it and only at the time of
planning we add the gating plan when there is a pseudo constant quals.
Why do we have different behaviours in these two cases? Following
comment in set_append_rel_size() doesn't explain why it's done for
child but not for parent.
1000 * The child rel's targetlist might contain non-Var
expressions, which
1001 * means that substitution into the quals could produce
opportunities
1002 * for const-simplification, and perhaps even pseudoconstant quals.
1003 * Therefore, transform each RestrictInfo separately to see if it
1004 * reduces to a constant or pseudoconstant. (We must process them
1005 * separately to keep track of the security level of each qual.)
1006 */
Why do we want to create paths for the relation which we know is not
going to produce any result?
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
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