Обсуждение: speed w/ OFFSET/LIMIT

Hi !

I'm running a pretty simple select query on a pretty large table (70000
records). This table has some "flag" fields, each one textually explained by
another table ( flag VARCHAR(2), flag_details VARCHAR(60))

SELECT t.* , t1.flag1_details , ... , tn.flagn_details
FROM table t
NATURAL JOIN t1
NATURAL JOIN ...
NATURAL JOIN tn
ORDER BY main_field OFFSET x LIMIT 50

There is no where statement, this query is for display purposes on a web page.
My problem is, where OFFSET is low, execution takes only a few milli-seconds,
but where OFFSET is high (50 last results for example), execution can take 10
seconds...
Is there anything I can do to speed up such a query ?

Try using subselects instead. PGSQL has to do all the joins unless you
do a LEFT JOIN so that it knows what rows should be included. It would
be nice if the optimizer could recognize if you're jointing to a table
that has RI, since it could safely transform to a subselect then, but
I'm pretty sure it doesn't do this.

On Tue, May 27, 2003 at 03:39:49PM +0200, Damien wrote:
> Hi !
>
> I'm running a pretty simple select query on a pretty large table (70000
> records). This table has some "flag" fields, each one textually explained by
> another table ( flag VARCHAR(2), flag_details VARCHAR(60))
>
> SELECT t.* , t1.flag1_details , ... , tn.flagn_details
> FROM table t
> NATURAL JOIN t1
> NATURAL JOIN ...
> NATURAL JOIN tn
> ORDER BY main_field OFFSET x LIMIT 50
>
> There is no where statement, this query is for display purposes on a web page.
> My problem is, where OFFSET is low, execution takes only a few milli-seconds,
> but where OFFSET is high (50 last results for example), execution can take 10
> seconds...
> Is there anything I can do to speed up such a query ?
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
>

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Damien wrote:
> Hi !
>
> I'm running a pretty simple select query on a pretty large table (70000
> records). This table has some "flag" fields, each one textually explained by
> another table ( flag VARCHAR(2), flag_details VARCHAR(60))
>
> SELECT t.* , t1.flag1_details , ... , tn.flagn_details
> FROM table t
> NATURAL JOIN t1
> NATURAL JOIN ...
> NATURAL JOIN tn
> ORDER BY main_field OFFSET x LIMIT 50
>
> There is no where statement, this query is for display purposes on a web page.
> My problem is, where OFFSET is low, execution takes only a few milli-seconds,
> but where OFFSET is high (50 last results for example), execution can take 10
> seconds...

This is because of the way OFFSET/LIMIT works internally. It basically
run's your query, throwing away (but fully creating) the first OFFSET
result rows, and aborting the query when reaching LIMIT+1 result rows
after that.


Jan

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