Re: Question about partitioned query behavior

Hi,

My question is regarding ORDER BY / LIMIT query behavior when using partitioning.

I have a large table (about 100 columns, several million rows) partitioned by a column called day (which is the date stored as yyyymmdd - say 20100502 for May 2nd 2010 etc.). Say the main table  is called FACT_TABLE and each child table is called FACT_TABLE_yyyymmdd (e.g. FACT_TABLE_20100502, FACT_TABLE_20100503 etc.) and has an appropriate CHECK constraint created on it to CHECK (day = yyyymmdd).

Postgres Version:  PostgreSQL 8.4.2 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC) 3.4.6 20060404 (Red Hat 3.4.6-10), 64-bit

The query pattern I am looking at is (I have tried to simplify the column names for readability):

SELECT F1 from FACT_TABLE
where day >= 20100502 and day <= 20100507  # selecting for a week
ORDER BY F2 desc
LIMIT 100


This is what is happening:

When I query from the specific day's (child) table, I get what I expect - a descending Index scan and good performance.

# explain  select F1 from FACT_TABLE_20100502 where day = 20100502 order by F2 desc limit 100;
                                                                    QUERY PLAN                                                                 
 
------------------------------------------------------------------------------------------------------------------------------------------------
--
 Limit  (cost=0.00..4.81 rows=100 width=41)
   ->  Index Scan Backward using F2_20100502 on FACT_TABLE_20100502  (cost=0.00..90355.89 rows=1876985 width=41
)
         Filter: (day = 20100502)



BUT:

When I do the same query against the parent table it is much slower - two things seem to happen - one is that the descending scan of the index is not done and secondly there seems to be a separate sort/limit at the end - i.e. all data from all partitions is retrieved and then sorted and limited - This seems to be much less efficient than doing a descending scan on each partition and limiting the results and then combining and reapplying the limit at the end.

explain  select F1 from FACT_TABLE where day = 20100502 order by F2 desc limit 100;
                                                                    QUERY PLAN                                                                 
  
------------------------------------------------------------------------------------------------------------------------------------------------
---
 Limit  (cost=20000084948.01..20000084948.01 rows=100 width=41)
   ->  Sort  (cost=20000084948.01..20000084994.93 rows=1876986 width=41)
         Sort Key: public.FACT_TABLE.F2
         ->  Result  (cost=10000000000.00..20000084230.64 rows=1876986 width=41)
               ->  Append  (cost=10000000000.00..20000084230.64 rows=1876986 width=41)
                     ->  Seq Scan on FACT_TABLE  (cost=10000000000.00..10000000010.02 rows=1 width=186)
                           Filter: (day = 20100502)
                     ->  Seq Scan on FACT_TABLE_20100502 FACT_TABLE  (cost=10000000000.00..10000084220.62 rows=1876985 width=4
1)
                           Filter: (day = 20100502)
(9 rows)


Could anyone please explain why this is happening and what I can do to get the query to perform well even when querying from the parent table?

Thanks,

Ranga

Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. See how.