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POOptimize SELECT query with ORDER BY, OFFSET and LIMIT of postgresql
 

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  1. POOptimize SELECT query with ORDER BY, OFFSET and LIMIT of postgresql
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    <p>This is my table schema</p> <pre><code>Column | Type | Modifiers -------------+------------------------+------------------------------------------------------ id | integer | not null default nextval('message_id_seq'::regclass) date_created | bigint | content | text | user_name | character varying(128) | user_id | character varying(128) | user_type | character varying(8) | user_ip | character varying(128) | user_avatar | character varying(128) | chatbox_id | integer | not null Indexes: "message_pkey" PRIMARY KEY, btree (id) "idx_message_chatbox_id" btree (chatbox_id) "indx_date_created" btree (date_created) Foreign-key constraints: "message_chatbox_id_fkey" FOREIGN KEY (chatbox_id) REFERENCES chatboxes(id) ON UPDATE CASCADE ON DELETE CASCADE </code></pre> <p>This is the query</p> <pre><code>SELECT * FROM message WHERE chatbox_id=$1 ORDER BY date_created OFFSET 0 LIMIT 20; </code></pre> <p>($1 will be replaced by the actual ID)</p> <p>It runs pretty well, but when it reaches 3.7 millions records, all SELECT queries start consuming a lot of CPU and RAM and then the whole system goes down. I have to temporarily backup all the current messages and truncate that table. I am not sure what is going on because everything is ok when I have about 2 millions records</p> <p>I am using Postresql Server 9.1.5 with default options. </p> <hr> <p>Update the output of EXPLAIN ANALYZE</p> <pre><code>Limit (cost=0.00..6.50 rows=20 width=99) (actual time=0.107..0.295 rows=20 loops=1) -&gt; Index Scan Backward using indx_date_created on message (cost=0.00..3458.77 rows=10646 width=99) (actual time=0.105..0.287 rows=20 loops=1) Filter: (chatbox_id = 25065) Total runtime: 0.376 ms (4 rows) </code></pre> <hr> <p>Update server specification</p> <pre><code>Intel Xeon 5620 8x2.40GHz+HT 12GB DDR3 1333 ECC SSD Intel X25-E Extreme 64GB </code></pre> <hr> <p><strong>Final solution</strong></p> <p>Finally I can go above 3 million messages, I have to optimize the postgresql configuration as wildplasser suggested and also make a new index as A.H. suggested</p>
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    1. USTan Nguyen
    1. USTan Nguyen
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    1. COCan you add `EXPLAIN ANALYZE` of the query?
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      1. POOptimize SELECT query with ORDER BY, OFFSET and LIMIT of postgresql
      1. USIgor Romanchenko
    2. CO`I am using Postresql Server 9.1.5 with default options.` You could start by setting estimated_cache_size to 3/4 of your available RAM, and setting work_mem to ~10M. And *maybe* set random_page_cost to 1.5. and run `VACUUM ANALYZE` on all relevant tables.
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      1. USwildplasser
    3. COHave you run an `ANALYZE` on the table recently?
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      1. POOptimize SELECT query with ORDER BY, OFFSET and LIMIT of postgresql
      1. USAndy Lester
 

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