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I use PostgreSQL 9.1 on Ubuntu 12.04.

I need to select records inside a range of time: my table time_limits has two timestamp fields and one integer property. There are additional columns in my actual table that are not involved with this query.

create table (
   start_date_time timestamp,
   end_date_time timestamp, 
   id_phi integer, 
   primary key(start_date_time, end_date_time,id_phi);

This table contains roughly 2M records.

Queries like the following took enormous amounts of time:

select * from time_limits as t 
where t.id_phi=0 
and t.start_date_time <= timestamp'2010-08-08 00:00:00'
and t.end_date_time   >= timestamp'2010-08-08 00:05:00';

So I tried adding another index - the inverse of the PK:

create index idx_inversed on time_limits(id_phi, start_date_time, end_date_time);

I got the impression that performance improved: The time for accessing records in the middle of the table seems to be more reasonable: somewhere between 40 and 90 seconds.

But it's still several tens of seconds for values in the middle of the time range. And twice more when targeting the end of the table (chronologically speaking).

I tried explain analyze for the first time to get this query plan:

 Bitmap Heap Scan on time_limits  (cost=4730.38..22465.32 rows=62682 width=36) (actual time=44.446..44.446 rows=0 loops=1)
   Recheck Cond: ((id_phi = 0) AND (start_date_time <= '2011-08-08 00:00:00'::timestamp without time zone) AND (end_date_time >= '2011-08-08 00:05:00'::timestamp without time zone))
   ->  Bitmap Index Scan on idx_time_limits_phi_start_end  (cost=0.00..4714.71 rows=62682 width=0) (actual time=44.437..44.437 rows=0 loops=1)
         Index Cond: ((id_phi = 0) AND (start_date_time <= '2011-08-08 00:00:00'::timestamp without time zone) AND (end_date_time >= '2011-08-08 00:05:00'::timestamp without time zone))
 Total runtime: 44.507 ms

See the results on depesz.com.

What could I do to optimize the search? You can see all the time is spent scanning the two timestamps columns once id_phi is set to 0. And I don't understand the big scan (60K rows!) on the timestamps. Aren't they indexed by the primary key and idx_inversed I added?

Should I change from timestamp types to something else?

I have read a little about GIST and GIN indexes. I gather they can be more efficient on certain conditions for custom types. Is it a viable option for my use case?

share|improve this question
    
45 milliseconds (that's 0.045 seconds) to search through 2 million rows seems quite OK to me. – a_horse_with_no_name Apr 9 '13 at 20:56
    
I think it is not 45 ms, but 45 s – jap1968 Apr 9 '13 at 21:01
1  
well it is 45s. I don't know why it says 45ms. I wouldn't even start complaining if that was as fast as 45ms... :-) Maybe a bug in the output of explain analyze. Or maybe it's the time of the analyze to perform. Dunno. But 40/50 seconds is what I measure. – Stephane Rolland Apr 9 '13 at 21:19
1  
The time reported in the explain analyze output is the time the query needed on the server. If your query takes 45 seconds, then the additional time is spent transferring the data from the database to the program running the query After all it's 62682 rows and if each row is large (e.g. has long varchar or text columns), this can impact the transfer time drastically. – a_horse_with_no_name Apr 9 '13 at 22:23
    
@a_horse_with_no_name: rows=62682 rowsis the planner's estimate. The query returns 0 rows. (actual time=44.446..44.446 rows=0 loops=1) – Erwin Brandstetter Apr 9 '13 at 23:23
up vote 43 down vote accepted

Solution for Postgres 9.1

CREATE INDEX idx_time_limits_inversed
ON time_limits (id_phi, start_date_time, end_date_time DESC);

In most cases the sort order of an index is hardly relevant. Postgres can scan backwards practically as fast. But for range queries on multiple columns it can make a huge difference. I wrote more in this closely related answer on SO.

Consider your query:

SELECT *
FROM   time_limits
WHERE  id_phi = 0
AND    start_date_time <= '2010-08-08 00:00'::timestamp
AND    end_date_time   >= '2010-08-08 00:05'::timestamp

Sort order of the first column id_phi in the index is irrelevant. Since it's checked for equality (=), it should come first. You got that right. More in this related answer:

Postgres can jump to id_phi = 0 in next to no time and consider the following two columns of the matching index. These are queried with range conditions of inverted sort order (<=, >=). In my index, qualifying rows come first. Should be the fastest possible way with a B-Tree index1:

  • You want start_date_time <= something: index has the earliest timestamp first.
    • If it qualifies, check column 3.
      Recurse until the first row fails to qualify (super fast).
  • You want end_date_time >= something: index has the latest timestamp first.
    • If it qualifies, keep fetching rows until the first one doesn't (super fast).
      Continue with next value for column 2 ..

Postgres can either scan forward or backward. The way you had the index, it has to read all rows matching on the first two columns and then filter on the third. Be sure to read the chapter Indexes and ORDER BY in the manual. It fits your question pretty well.

How many rows match on the first two columns?
Only few with a start_date_time close to the start of the time range of the table. But almost all rows with id_phi = 0 at the chronological end of the table! So performance deteriorates with later start times.

Planner estimates

The planner estimates rows=62682 for your example query. Of those, none qualify (rows=0). You might get better estimates if you increase the statistics target for the table. For 2.000.000 rows ...

ALTER TABLE time_limits ALTER start_date_time SET STATISTICS 1000;
ALTER TABLE time_limits ALTER end_date_time   SET STATISTICS 1000;

... might pay. Or even higher. More in this related answer on SO. My guess would be you don't need that for id_phi (only few distinct values, evenly distributed), but for the timestamps (lots of distinct values, unevenly distributed).
I also don't think it matters much with the improved index.

CLUSTER / pg_repack

If you want it faster, yet, you could streamline the physical order of rows in your table. If you can afford to lock your table exclusively for a few seconds (at off hours for instance) to rewrite your table and order rows according to the index:

ALTER TABLE time_limits CLUSTER ON idx_time_limits_inversed;

If you have concurrent access, consider pg_repack, which can do the same without exclusive lock.

Either way, the effect is that fewer blocks need to be read from the table and everything is pre-sorted. It's a one-time effect deteriorating over time, if you have writes on the table.

GiST index in Postgres 9.2+

1 With pg 9.2+ there is another, possibly faster option: a GiST index for a range column.

  • There are built-in range types for timestamp and timestamp with time zone: tsrange, tstzrange. A btree index is normally faster for the additional integer column id_phi, smaller and cheaper to maintain, too. But the query would probably still be faster overall with the combined index.

  • Change your table definition or make that a functional index.

  • For the multicolumn index at hand you also need the additional module btree_gist (once per database) to include the integer in your GiST index.

The trifecta! A multicolumn functional GiST index:

CREATE EXTENSION IF NOT EXISTS btree_gist;  -- only if not installed, yet.

CREATE INDEX idx_time_limits_funky ON time_limits USING gist
(id_phi, tsrange(start_date_time, end_date_time, '[]'));

Use the "contains range" operator @> in your query now:

SELECT *
FROM   time_limits
WHERE  id_phi = 0
AND    tsrange(start_date_time, end_date_time, '[]')
    @> tsrange('2010-08-08 00:00', '2010-08-08 00:05', '[]')
share|improve this answer
17  
I should tell this at least only once, that each of your answers on SO and DBA are of really high added value/exprertise, and most of the time the most complete. Just to say it once: Respect!. – Stephane Rolland Apr 10 '13 at 2:14
    
Merci bien! :) So did you get faster results? – Erwin Brandstetter Apr 10 '13 at 2:34
    
I have to let finish the big bulk copy generated from the intensively awkward query of mine, so making the process really slow, it was turning for hours before I asked the question. But I have calculated, and I decided to let it turn until tomorow morning, it'll be finished, and the new table ready to be filled tomorow. I have tried to create your index concurrently during the job, but due to too much access(i think), the creation of the index should be locked. I'll repeat this same test time again tomorow with your solution. I have also looked at how upgrading to 9.2 ;-) for debian/ubuntu. – Stephane Rolland Apr 10 '13 at 3:29
    
the tsrange type really recovers the data I want to encode. It's only with your explication that I grasped that the database had no idea of the relative order between the two different timestamp column. I'll welcome 9.2. – Stephane Rolland Apr 10 '13 at 3:39
2  
@StephaneRolland: it would still be interesting why the explain analyze output shows 45milliseconds while you see the query taking over 40 seconds. – a_horse_with_no_name Apr 10 '13 at 7:19

Erwin's answer is already comprehensive, however:

Range types for timestamps are available in PostgreSQL 9.1 with the Temporal extension from Jeff Davis: https://github.com/jeff-davis/PostgreSQL-Temporal

Note: has limited features (uses Timestamptz, and you can only have the '[)' style overlap afaik). Also, there's lots of other great reasons to upgrade to PostgreSQL 9.2.

share|improve this answer

You could try to create the multicolumn index in a different order:

primary key(id_phi, start_date_time,end_date_time);

I posted once a similar question also related to the ordering of indexes on a multicolumn index. The key is trying to use first the most restrictive conditions to reduce the search space.

Edit: My mistake. Now I see that you already have this index defined.

share|improve this answer
    
I already have both index. Except the primary key is the other, but the index you propose already exists, and is the one that is used if you look at the explain: Bitmap Index Scan on idx_time_limits_phi_start_end – Stephane Rolland Apr 9 '13 at 20:29

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