Thread: 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

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