Postgres Slow group by query with max

Question

I am using postgres 9.1 and I have a table with about 3.5M rows of eventtype (varchar) and eventtime (timestamp) - and some other fields. There are only about 20 different

Answer 1

Based on the question you already have the relevant index.

If upgrading to Postgres 9.3 or an index on (eventtype, eventtime desc) doesn't make a difference, this is a case where rewriting the query so it uses a correlated subquery works very well if you can enumerate all of the event types manually:

select val as eventtype,
       (select max(eventtime)
        from allevents
        where allevents.eventtype = val
        ) as eventtime
from unnest('{type1,type2,…}'::text[]) as val;

---

Here's the plans I get when running similar queries:

denis=# select version();
                                                              version                                                              
-----------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.3.1 on x86_64-apple-darwin11.4.2, compiled by Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn), 64-bit
(1 row)

Test data:

denis=# create table test (evttype int, evttime timestamp, primary key (evttype, evttime));
CREATE TABLE
denis=# insert into test (evttype, evttime) select i, now() + (i % 3) * interval '1 min' - j * interval '1 sec' from generate_series(1,10) i, generate_series(1,10000) j;
INSERT 0 100000
denis=# create index on test (evttime, evttype);
CREATE INDEX
denis=# vacuum analyze test;
VACUUM

First query:

denis=# explain analyze select evttype, max(evttime) from test group by evttype;                                                    QUERY PLAN                                                     
-------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=2041.00..2041.10 rows=10 width=12) (actual time=54.983..54.987 rows=10 loops=1)
   ->  Seq Scan on test  (cost=0.00..1541.00 rows=100000 width=12) (actual time=0.009..15.954 rows=100000 loops=1)
 Total runtime: 55.045 ms
(3 rows)

Second query:

denis=# explain analyze select val as evttype, (select max(evttime) from test where test.evttype = val) as evttime from unnest('{1,2,3,4,5,6,7,8,9,10}'::int[]) val;
                                                                        QUERY PLAN                                                                         
-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Function Scan on unnest val  (cost=0.00..48.39 rows=100 width=4) (actual time=0.086..0.292 rows=10 loops=1)
   SubPlan 2
     ->  Result  (cost=0.46..0.47 rows=1 width=0) (actual time=0.024..0.024 rows=1 loops=10)
           InitPlan 1 (returns $1)
             ->  Limit  (cost=0.42..0.46 rows=1 width=8) (actual time=0.021..0.021 rows=1 loops=10)
                   ->  Index Only Scan Backward using test_pkey on test  (cost=0.42..464.42 rows=10000 width=8) (actual time=0.019..0.019 rows=1 loops=10)
                         Index Cond: ((evttype = val.val) AND (evttime IS NOT NULL))
                         Heap Fetches: 0
 Total runtime: 0.370 ms
(9 rows)