I have been fooling around with adding decompiled display of plan
qualification conditions to EXPLAIN output. With this, you can
for example tell the difference between indexscanned and
not-indexscanned clauses, without having to dig through EXPLAIN
VERBOSE dumps. Here is an example motivated by Rob Hoopman's
recent query on pgsql-general:
regression=# create table foo (f1 int, f2 int, f3 int, unique(f1,f2));
NOTICE: CREATE TABLE / UNIQUE will create implicit index 'foo_f1_key' for table 'foo'
CREATE
regression=# explain select * from foo where f1 = 11;
INFO: QUERY PLAN:
Index Scan using foo_f1_key on foo (cost=0.00..17.07 rows=5 width=12) indxqual: (f1 = 11)
EXPLAIN
regression=# explain select * from foo where f1 = 11 and f2 = 44;
INFO: QUERY PLAN:
Index Scan using foo_f1_key on foo (cost=0.00..4.83 rows=1 width=12) indxqual: ((f1 = 11) AND (f2 = 44))
EXPLAIN
regression=# explain select * from foo where f1 = 11 and f3 = 44;
INFO: QUERY PLAN:
Index Scan using foo_f1_key on foo (cost=0.00..17.08 rows=1 width=12) indxqual: (f1 = 11) qual: (f3 = 44)
EXPLAIN
regression=# explain select * from foo where f2 = 11 and f3 = 44;
INFO: QUERY PLAN:
Seq Scan on foo (cost=0.00..25.00 rows=1 width=12) qual: ((f2 = 11) AND (f3 = 44))
EXPLAIN
The display of join conditions isn't yet ready for prime time:
regression=# explain select * from tenk1 a left join tenk1 b using (unique1)
regression-# where a.hundred < b.hundred;
INFO: QUERY PLAN:
Merge Join (cost=0.00..2343.45 rows=10000 width=296) merge: ("outer"."?column1?" = "inner"."?column16?") qual:
("outer"."?column7?"< "inner"."?column6?") -> Index Scan using tenk1_unique1 on tenk1 a (cost=0.00..1071.78
rows=10000width=148) -> Index Scan using tenk1_unique1 on tenk1 b (cost=0.00..1071.78 rows=10000 width=148)
EXPLAIN
but it's getting there.
Question for the group: does this seem valuable enough to put into the
standard EXPLAIN output, or should it be a special option? I can
imagine showing it only in EXPLAIN VERBOSE's summary display, or adding
a GUC variable to enable it, or adding another option keyword to
EXPLAIN, but I don't much want to do any of those things. On the other
hand, maybe this stuff won't make any sense to non-experts anyway.
Thoughts?
regards, tom lane
