query partitioned table is very slow

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I have about 900 partitioned tables with 67 millons rows.
And I found that my query takes too much time!
------------------------------------------------------------------------------
explain  ( ANALYZE,VERBOSE,BUFFERS )
 select report_id from cars."all"
    WHERE
    report_datetime  = '2015-10-14 00:02:02+03'::timestamptz   AND
    report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid
------------------------------------------------------------------------------
Append (cost=0.00..4.43 rows=2 width=4) (actual time=0.023..0.023 rows=1 loops=1)
  Buffers: shared hit=4
-> Seq Scan on cars.all (cost=0.00..0.00 rows=1 width=4) (actual time=0.002..0.002 rows=0 loops=1)
        Output: all.report_id
Filter: ((all.report_datetime = '2015-10-14 00:02:02+03'::timestamp with time zone) AND (all.report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid)) -> Index Scan using day_151014_uuid_idx on cars.day_151014 (cost=0.42..4.43 rows=1 width=4) (actual time=0.020..0.020 rows=1 loops=1)
        Output: day_151014.report_id
Index Cond: (day_151014.report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid) Filter: (day_151014.report_datetime = '2015-10-14 00:02:02+03'::timestamp with time zone)
        Buffers: shared hit=4
Total runtime: 0.096 ms
------------------------------------------------------------------------------

This query takes about 500ms. But query from only part-table takes 12ms:

select report_id from cars.day_151014
    WHERE
    report_datetime  = '2015-10-14 00:02:02+03'::timestamptz   AND
    report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid
------------------------------------------------------------------------------
 explain  ( ANALYZE,VERBOSE,BUFFERS )
 select report_id from cars.day_151014
    WHERE
    report_datetime  = '2015-10-14 00:02:02+03'::timestamptz   AND
    report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid
------------------------------------------------------------------------------
Index Scan using day_151014_uuid_idx on cars.day_151014 (cost=0.42..4.43 rows=1 width=4) (actual time=0.022..0.023 rows=1 loops=1)
  Output: report_id
Index Cond: (day_151014.report_uuid = 'f6b08f92-0d5d-28b0-81c3-0c20c4ca3038'::uuid) Filter: (day_151014.report_datetime = '2015-10-14 00:02:02+03'::timestamp with time zone)
  Buffers: shared hit=4
Total runtime: 0.045 ms
------------------------------------------------------------------------------

Query plans seems fine, but why actual query is so slow?

p.s. PostgreSQL 9.3.9 x86_64 on Oracle Linux Server release 6.6 (3.8.13-68.3.2.el6uek.x86_64)





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