SQL Performance Help

I'm developing a course calendar for a client and I'm running into
performance problems with the admin site. For example, when I try to include
registration counts in the course list, the page really slows down for large
course lists (50 or so):

COURSE ATTENDEES CAPACITY SEATS LEFT
====== ========= ======== ==========
Course 1 5 10 5
Course 2 6 15 9
Course 3 4 10 6

I've been using one query to retrieve the course list and then one for each
attendee count. Is there a more efficient way of doing this all in one
query? I was thinking something like this (I'm not a SQL expert, so I don't
know if this is even possible):

SELECT
course_name,
capacity,
count(query here) as attendee_count
FROM events AS e
LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
WHERE start_time BETWEEN point_a AND point_b

Or should I just pull everything as a separate row like this and sort it all
out programmatically:

SELECT
e.course_name,
e.capacity,
a.user_id
FROM events AS e
LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
WHERE start_time BETWEEN point_a AND point_b

Or should I just try caching the data in PHP? Would an index help?

I realize any answers might be complicated, but if you could just point me
in the right direction, I can probably figure the rest out.

Thanks,
Tony

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RE: SQL Performance Help

1. yes indexes could help, if mysql uses them. The mysql optimiser may or
may not use the index for the query depending on the statement...it sounds
like you are doing a full table scan on the data

2. there are two schools of thought here:
a. run the whole thing as two statements (one outer loop to loop thru the
course list, and an inner one to get the attendees)
b. examine the join query to see if you are starting out in the correct way
(mysql can be sensitive to the way the tables are joined), examine the
indexes and use the EXPLAIN statement to see how the optimiser attempts the
query.

I would just test both ways and see which one performs better...

Bastien


>From: Tony Grimes
>To: PHP-DB
>Subject: [PHP-DB] SQL Performance Help
>Date: Wed, 27 Dec 2006 14:05:13 -0700
>
>I'm developing a course calendar for a client and I'm running into
>performance problems with the admin site. For example, when I try to
>include
>registration counts in the course list, the page really slows down for
>large
>course lists (50 or so):
>
>COURSE ATTENDEES CAPACITY SEATS LEFT
>====== ========= ======== ==========
>Course 1 5 10 5
>Course 2 6 15 9
>Course 3 4 10 6
>
>I've been using one query to retrieve the course list and then one for each
>attendee count. Is there a more efficient way of doing this all in one
>query? I was thinking something like this (I'm not a SQL expert, so I don't
>know if this is even possible):
>
>SELECT
> course_name,
> capacity,
> count(query here) as attendee_count
>FROM events AS e
>LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
>WHERE start_time BETWEEN point_a AND point_b
>
>Or should I just pull everything as a separate row like this and sort it
>all
>out programmatically:
>
>SELECT
> e.course_name,
> e.capacity,
> a.user_id
>FROM events AS e
>LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
>WHERE start_time BETWEEN point_a AND point_b
>
>Or should I just try caching the data in PHP? Would an index help?
>
>I realize any answers might be complicated, but if you could just point me
>in the right direction, I can probably figure the rest out.
>
>Thanks,
>Tony
>
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>PHP Database Mailing List (http://www.php.net/)
>To unsubscribe, visit: http://www.php.net/unsub.php
>

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Re: SQL Performance Help

Tony Grimes wrote:
> I'm developing a course calendar for a client and I'm running into
> performance problems with the admin site. For example, when I try to include
> registration counts in the course list, the page really slows down for large
> course lists (50 or so):
>
> COURSE ATTENDEES CAPACITY SEATS LEFT
> ====== ========= ======== ==========
> Course 1 5 10 5
> Course 2 6 15 9
> Course 3 4 10 6
>
> I've been using one query to retrieve the course list and then one for each
> attendee count. Is there a more efficient way of doing this all in one
> query? I was thinking something like this (I'm not a SQL expert, so I don't
> know if this is even possible):
>
> SELECT
> course_name,
> capacity,
> count(query here) as attendee_count
> FROM events AS e
> LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
> WHERE start_time BETWEEN point_a AND point_b
>
> Or should I just pull everything as a separate row like this and sort it all
> out programmatically:
>
> SELECT
> e.course_name,
> e.capacity,
> a.user_id
> FROM events AS e
> LEFT OUTER JOIN event_attendees AS a ON e.event_id = a.event_id
> WHERE start_time BETWEEN point_a AND point_b
>
> Or should I just try caching the data in PHP? Would an index help?

Index your tables, make the database do the work. Much easier and less
prone to bugs :)

Check you have an index on:

events(event_id)
event_attendees(event_id)

table(start_time) (whichever that table applies to - I assume it's events).

Maybe try a multi-column index if this query gets run a lot:

create index event_eventid_start_time on events(event_id, start_time);

Use 'explain' to see which one is being used and possibly get rid of the
other one.


I have a guide about how to index databases here:

http://www.designmagick.com/article/16/

(Yes it's a postgresql site but the same rules apply to mysql and other
databases as well).

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