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nested query on last n rows of huge table

From: "jcfischer" <jcfischer(dot)lists(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: nested query on last n rows of huge table
Date: 2006-02-24 14:13:31
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Hi list

I'm fairly new to Postgres so bear with me. Googling and searching the
list, I didn't find anything that resembled my problem.

I have a large table with ca. 10 million inserts per day (fairly simple
data: timestam, a couple of id's and a varchar message)

I run a query every couple of minutes that looks at the new entries
since the last run and retrieves them for further processing (using a
WHERE eventtime > '2006-02-24 14:00:00' ) to limit to the most recent

These queries run around 40-50 seconds (largely due to some LIKE %msg%
threwn in for good measure). Postgres performs a seq table scan on
those queries :-(

My idea is to limit the search to only the last n entries because I
found that a

SELECT * from table ORDER eventtime DESC limit 1000

is very fast. Because the inserts are in chronolgical order, I can
store the sequential id of the highest row from the last query and
subtract that from the current high row count to determine that number.

Is there a way to limit the expensive query to only those last 1000 (or
whatever) results?

I have tried to nest SELECTS but my SQL-fu is to limited to get
anything through the SQL processor :-)

Jens-Christian Fischer


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Next:From: jcfischerDate: 2006-02-24 14:26:40
Subject: Re: nested query on last n rows of huge table
Previous:From: Brendan DuddridgeDate: 2006-02-24 07:24:00
Subject: Re: Really really slow query. What's a better way?

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