Improve performance of first query

Question

If the following database (postgres) queries are executed, the second call is much faster.

I guess the first query is slow since the operating system (linux) needs to get the data from disk. The second query benefits from caching at filesystem level and in postgres.

Is there a way to optimize the database to get the results fast on the first call?

First call (slow)

foo3_bar_p@BAR-FOO3-Test:~$ psql

foo3_bar_p=# explain analyze SELECT "foo3_beleg"."id", ... FROM "foo3_beleg" WHERE 
foo3_bar_p-# (("foo3_beleg"."id" IN (SELECT beleg_id FROM foo3_text where 
foo3_bar_p(# content @@ 'footown'::tsquery)) AND "foo3_beleg"."belegart_id" IN 
foo3_bar_p(# ('...', ...));
                                                                                             QUERY PLAN                                                                                 
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=75314.58..121963.20 rows=152 width=135) (actual time=27253.451..88462.165 rows=11 loops=1)
   ->  HashAggregate  (cost=75314.58..75366.87 rows=5229 width=4) (actual time=16087.345..16113.988 rows=17671 loops=1)
         ->  Bitmap Heap Scan on foo3_text  (cost=273.72..75254.67 rows=23964 width=4) (actual time=327.653..16026.787 rows=27405 loops=1)
               Recheck Cond: (content @@ '''footown'''::tsquery)
               ->  Bitmap Index Scan on foo3_text_content_idx  (cost=0.00..267.73 rows=23964 width=0) (actual time=281.909..281.909 rows=27405 loops=1)
                     Index Cond: (content @@ '''footown'''::tsquery)
   ->  Index Scan using foo3_beleg_pkey on foo3_beleg  (cost=0.00..8.90 rows=1 width=135) (actual time=4.092..4.092 rows=0 loops=17671)
         Index Cond: (id = foo3_text.beleg_id)
         Filter: ((belegart_id)::text = ANY ('{...
         Rows Removed by Filter: 1
 Total runtime: 88462.809 ms
(11 rows)

Second call (fast)

  Nested Loop  (cost=75314.58..121963.20 rows=152 width=135) (actual time=127.569..348.705 rows=11 loops=1)
   ->  HashAggregate  (cost=75314.58..75366.87 rows=5229 width=4) (actual time=114.390..133.131 rows=17671 loops=1)
         ->  Bitmap Heap Scan on foo3_text  (cost=273.72..75254.67 rows=23964 width=4) (actual time=11.961..97.943 rows=27405 loops=1)
               Recheck Cond: (content @@ '''footown'''::tsquery)
               ->  Bitmap Index Scan on foo3_text_content_idx  (cost=0.00..267.73 rows=23964 width=0) (actual time=9.226..9.226 rows=27405 loops=1)
                     Index Cond: (content @@ '''footown'''::tsquery)
   ->  Index Scan using foo3_beleg_pkey on foo3_beleg  (cost=0.00..8.90 rows=1 width=135) (actual time=0.012..0.012 rows=0 loops=17671)
         Index Cond: (id = foo3_text.beleg_id)
         Filter: ((belegart_id)::text = ANY ('...
         Rows Removed by Filter: 1
 Total runtime: 348.833 ms
(11 rows)

Table layout of the foo3_text table (28M rows)

foo3_egs_p=# \d foo3_text
                                 Table "public.foo3_text"
  Column  |         Type          |                         Modifiers                          
----------+-----------------------+------------------------------------------------------------
 id       | integer               | not null default nextval('foo3_text_id_seq'::regclass)
 beleg_id | integer               | not null
 index_id | character varying(32) | not null
 value    | text                  | not null
 content  | tsvector              | 
Indexes:
    "foo3_text_pkey" PRIMARY KEY, btree (id)
    "foo3_text_index_id_2685e3637668d5e7_uniq" UNIQUE CONSTRAINT, btree (index_id, beleg_id)
    "foo3_text_beleg_id" btree (beleg_id)
    "foo3_text_content_idx" gin (content)
    "foo3_text_index_id" btree (index_id)
    "foo3_text_index_id_like" btree (index_id varchar_pattern_ops)
Foreign-key constraints:
    "beleg_id_refs_id_6e6d40770e71292" FOREIGN KEY (beleg_id) REFERENCES foo3_beleg(id) DEFERRABLE INITIALLY DEFERRED
    "index_id_refs_name_341600137465c2f9" FOREIGN KEY (index_id) REFERENCES foo3_index(name) DEFERRABLE INITIALLY DEFERRED

Hardware changes (SSD instead of traditional disks) or RAM disks are possible. But maybe there the current hardware can do faster results, too.

Version: PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu

Please leave a comment if you need more details.

Answer 1

Postgres is providing you a chance to do some configuration on runtime query executing for deciding your I/O operation priority.

random_page_cost(floating point) -(reference) is what may help you. It will basically set your IO/CPU operation ratio.

Higher value means I/O is important, I have sequential disk; and lower value means I/O is not important, I have random-access disk.

Default value is 4.0, and may be you want to increase and test if your query take shorter time.

Do not forget, your I/O priority will depend on your column count, row count.

A big BUT; since your indicies are btree, your CPU priority is going down much faster than I/O priorities going up. You can basically map complexities to priorities.

CPU Priority = O(log(x))
I/O Priority = O(x)

All in all, this means, if Postgre's value 4.0 would for 100k entries, You should set it to (approx.) (4.0 * log(100k) * 10M)/(log(10M) * 100k) for 10M entry.

Answer 2

Agree with Julius but, if you only need stuff from foo3_beleg, try EXISTS in instead (and it would help if you'd pasted your sql too, not just your explain plan).

select ...
from foo3_beleg b
where exists
(select 1 from foo_text s where t.beleg_id = b.id)
....

However, I suspect your "wake up" on the 1st pass is just your db loading up the IN subquery rows into memory. That will likely happen regardless, though an EXISTS is generally much faster than an IN (INs are rarely needed, if not containing hardcoded lists, and a yellow flag if I review sql).

Answer 3

The first time you execute the query, postgres will load the data from the disk which is slow even with a good hard drive. The second time you run your query it will load the previously loaded data from the RAM which is obviously faster.

The solution to this problem would be to load relation data into either the operating system buffer cache or the PostgreSQL buffer cache with:

int8 pg_prewarm(regclass, mode text default 'buffer', fork text default 'main', first_block int8 default null, last_block int8 default null) :

The first argument is the relation to be prewarmed. The second argument is the prewarming method to be used, as further discussed below; the third is the relation fork to be prewarmed, usually main. The fourth argument is the first block number to prewarm (NULL is accepted as a synonym for zero). The fifth argument is the last block number to prewarm (NULL means prewarm through the last block in the relation). The return value is the number of blocks prewarmed.

There are three available prewarming methods. prefetch issues asynchronous prefetch requests to the operating system, if this is supported, or throws an error otherwise. read reads the requested range of blocks; unlike prefetch, this is synchronous and supported on all platforms and builds, but may be slower. buffer reads the requested range of blocks into the database buffer cache.

Note that with any of these methods, attempting to prewarm more blocks than can be cached — by the OS when using prefetch or read, or by PostgreSQL when using buffer — will likely result in lower-numbered blocks being evicted as higher numbered blocks are read in. Prewarmed data also enjoys no special protection from cache evictions, so it is possible for other system activity may evict the newly prewarmed blocks shortly after they are read; conversely, prewarming may also evict other data from cache. For these reasons, prewarming is typically most useful at startup, when caches are largely empty.

Source

Hope this helped !