Re: LIMIT confuses the planner (again)

Robert Haas <robertmhaas@xxxxxxxxx> · Mon, 28 Sep 2009 07:38:54 -0400

On Mon, Sep 28, 2009 at 4:43 AM, Kouber Saparev <kouber@xxxxxxxxxxx> wrote:
> Hello,
>
> I am using PostgreSQL 8.3.7 and I am experiencing an issue similar to the
> one I've already described some time ago:
> http://archives.postgresql.org/pgsql-performance/2009-02/msg00261.php
>
> Again, adding a LIMIT clause to a query, which is normally executing very
> fast thanks to an index, makes it perform slow, because the planner no
> longer uses the "correct" index.
>
> I have the following table:
>
> CREATE TABLE message (
>  message_sid SERIAL PRIMARY KEY,
>  from_profile_sid INT NOT NULL REFERENCES profile,
>  to_profile_sid INT NOT NULL REFERENCES profile,
>  sender_has_deleted BOOLEAN NOT NULL DEFAULT FALSE,
>  receiver_has_deleted BOOLEAN NOT NULL DEFAULT FALSE,
>  body TEXT,
>  datetime TIMESTAMP NOT NULL DEFAULT NOW()
> );
>
>
> With the following conditional index:
>
> CREATE INDEX message_to_profile_idx ON message (to_profile_sid) WHERE NOT
> receiver_has_deleted;
>
>
> The query to obtain the list of received messages of a profile is simple and
> executes very fast, because of the index above:
>
> db=# EXPLAIN ANALYZE SELECT
>  *
> FROM
>  message
> WHERE
>  to_profile_sid = -1
> AND
>  NOT receiver_has_deleted
> ORDER BY
>  message_sid DESC;
>                                                                QUERY PLAN
> -------------------------------------------------------------------------------------------------------------------------------------------
>  Sort  (cost=11857.09..11866.19 rows=3640 width=277) (actual
> time=0.317..0.319 rows=15 loops=1)
>   Sort Key: message_sid
>   Sort Method:  quicksort  Memory: 32kB
>   ->  Bitmap Heap Scan on message  (cost=106.44..11641.78 rows=3640
> width=277) (actual time=0.096..0.271 rows=15 loops=1)
>         Recheck Cond: ((to_profile_sid = (-1)) AND (NOT
> receiver_has_deleted))
>         ->  Bitmap Index Scan on message_to_profile_idx (cost=0.00..105.53
> rows=3640 width=0) (actual time=0.056..0.056 rows=21 loops=1)
>               Index Cond: (to_profile_sid = (-1))
>  Total runtime: 0.383 ms
> (8 rows)
>
>
> Adding a LIMIT clause to exactly the same query slows its execution more
> than 20'000 times:
>
> db=# EXPLAIN ANALYZE SELECT
>  *
> FROM
>  message
> WHERE
>  to_profile_sid = -1
> AND
>  NOT receiver_has_deleted
> ORDER BY
>  message_sid DESC LIMIT 20;
>
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------------------------
>  Limit  (cost=0.00..6513.60 rows=20 width=277) (actual time=0.617..6576.539
> rows=15 loops=1)
>   ->  Index Scan Backward using message_pkey on message
> (cost=0.00..1185474.32 rows=3640 width=277) (actual time=0.617..6576.522
> rows=15 loops=1)
>         Filter: ((NOT receiver_has_deleted) AND (to_profile_sid = (-1)))
>  Total runtime: 6576.572 ms
> (4 rows)
>
>
> Just as I was advised in my recent post, I've already increased the
> statistics of both fields all the way till 1000, analyzed the table and
> reindexed the index:
>
> ALTER TABLE message ALTER COLUMN to_profile_sid SET STATISTICS 1000;
> ALTER TABLE message ALTER COLUMN receiver_has_deleted SET STATISTICS 1000;
> ANALYZE message;
> REINDEX index message_to_profile_idx;
>
>
> This time, however, the steps above didn't affect the planner in any way, it
> still refuses to use the index "message_to_profile_idx" when a LIMIT is
> involved (for this particular value of to_profile_sid).
>
> Here's some statistical data:
>
> db=# SELECT COUNT(*) FROM message;
>  count
> ---------
>  1312213
> (1 row)
>
> db=# SELECT COUNT(*) FROM message WHERE to_profile_sid = -1;
>  count
> -------
>  5604
> (1 row)
>
> db=# SELECT COUNT(*) FROM message WHERE to_profile_sid = -1 AND NOT
> receiver_has_deleted;
>  count
> -------
>    15
> (1 row)
>
> db=# SELECT COUNT(DISTINCT to_profile_sid) FROM message;
>  count
> -------
>  8596
> (1 row)
>
> db=# SELECT AVG(length) FROM (SELECT to_profile_sid, COUNT(*) AS length FROM
> message GROUP BY to_profile_sid) AS freq;
>         avg
> ----------------------
>  152.6540251279664960
> (1 row)
>
> db=# SELECT n_distinct FROM pg_stats WHERE tablename='message' AND
> attname='to_profile_sid';
>  n_distinct
> ------------
>       6277
> (1 row)
>
>
> Also, the value of -1 for "to_profile_sid" is second in the list of
> most_common_vals in pg_stats, but still I don't understand why a simple
> limit is blinding the planner for the "good" index. Any ideas?

It would be good to see what the planner's second choice would be, if
it didn't have that other index.

BEGIN;
DROP INDEX message_pkey;
EXPLAIN ANALYZE ...
ROLLBACK;

However, I suspect what's going on here is as follows.  When trying to
estimate the cost of LIMIT, the planner takes the startup cost for the
subpath and a pro-rata share of the run cost, based on the number of
rows being fetched as a fraction of the total number it believes to be
present.  So if the run cost is estimated to be lower than it really
is, some other plan with a lower startup cost can look like a better
choice, even if the run cost is much higher (because only a tiny
fraction of the run cost is being counted).

The reason why the run cost is being misestimated is because the
planner is estimating that the fraction of rows where to_profile_sid =
-1 and NOT receiver_has_deleted is equal to the fraction where
to_profile_sid = -1 multiplied by the fraction where NOT
received_has_deleted - and it isn't.  You might try creating a partial
index on message_sid WHERE NOT receiver_has_deleted, and see if that
helps.