Baron Schwartz wrote:
Chris Hemmings wrote:
Chris Hemmings wrote:
Baron Schwartz wrote:
Hi Chris,
Chris Hemmings wrote:
Dan Buettner wrote:
Chris, a couple of thoughts -
First, your index on the section is doing you no good (at this
time) since
all the values are the same. You may already know that, but
thought I'd
mention it.
Second, my theory on why query #1 is faster - if all your prices
range from
1 up, and you're querying for prices greater than 0, then MySQL
can just
return the first 30 rows after sorting them.
The second query, where you are looking for prices greater than
1, MySQL has
to sort and then examine a number of rows until it finds enough
matching
rows (price > 1) to satisfy your query. This likely takes a
little bit of
time. How many rows do you have with price = 1? It would have
to scan over
that many before it could start satisfying your query, if you
think about
it.
HTH,
Dan
On 9/10/07, Chris Hemmings <[EMAIL PROTECTED]> wrote:
Hello,
I have a table, currently holding 128,978 rows... In this
table, I have a
section column (int) and a price column (int). Every row has a
section of
1
currently, every row has a price, ranging from 1 to 10,000.
I have an index on both columns separately.
Have a look at these two queries, can someone tell me why there
is such a
difference in speed of execution? (Note difference in price
qualifier)
########################################
SELECT *
FROM `table1`
WHERE price >0
AND section =1
ORDER BY price
LIMIT 0 , 30
Showing rows 0 - 29 (128,978 total, Query took 0.9462 sec)
Explain output: 1 SIMPLE table1 ALL section,price NULL NULL NULL
96734
Using
where; Using filesort
########################################
SELECT *
FROM `table1`
WHERE price >1
AND section =1
ORDER BY price
LIMIT 0 , 30
Showing rows 0 - 29 (128,949 total, Query took 0.0008 sec)
Explain output: 1 SIMPLE table1 ALL section,price NULL NULL NULL
96734
Using
where; Using filesort
########################################
Other info:
Query cacheing = off
MySQL version = 5.0.32
OS = Debian Sarge
Sure, the second query returns 29 fewer records than the first,
but should
that make the difference in time?
Hope you can shed some light onto this :-)
Ta!
Chris.
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Thanks Dan,
I've got you on the section index... I was going to use that
later, when I get somre real data in there.
Anyway, I agree with your logic, but, the inverse is happening.
The one where it has to actually exclude some rows (because
price>1) is actually faster. Thats really why this has me
baffled, I would presume that the price>1 would be slower as it
does have to filter rows out first.
There's an easy way to find out: FLUSH STATUS, run the query, SHOW
STATUS LIKE 'handler%'. Do this on an otherwise quiet server if
possible.Or use MySQL Query Profiler -- it does a lot of math for
you :-)Baron
Thanks Baron!
I think you have hit upon something, doing what you said on a
'silent' server, I get the following:
SELECT * FROM `table1` WHERE price >1 AND section =1 ORDER BY price
LIMIT 0 , 30;
30 rows in set (0.00 sec)
mysql> SHOW STATUS LIKE 'handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 0 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 0 |
| Handler_read_key | 1 |
| Handler_read_next | 29 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 0 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_update | 0 |
| Handler_write | 14 |
+----------------------------+-------+
15 rows in set (0.00 sec)
SELECT * FROM `table1` WHERE price >0 AND section =1 ORDER BY price
LIMIT 0 , 30;
30 rows in set (0.95 sec)
mysql> SHOW STATUS LIKE 'handler%';
+----------------------------+--------+
| Variable_name | Value |
+----------------------------+--------+
| Handler_commit | 0 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 0 |
| Handler_read_key | 1 |
| Handler_read_next | 128978 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 30 |
| Handler_read_rnd_next | 0 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_update | 0 |
| Handler_write | 14 |
+----------------------------+--------+
15 rows in set (0.01 sec)
So, the slower query obvisouly has the larger Handler_read_next
number in it. Looking at the manual, it says the following for that
variable:
"The number of requests to read the next row in key order. This
value is incremented if you are querying an index column with a
range constraint or if you are doing an index scan."
So, being a bit of a MySQL novice, I can't really see how >0
increases the number of reads by such a VAST amount... Can you guys
put this in layman's terms for me?
Thanks for the help guys :-)
Chris.
Hi Guys,
I'm still not having much luck with this, but, I have spotted
something that maybe I missed earlier.
Doing an explain on both queries again produces the following:
explain select * from table1 where price>0 and section=1 order by
price limit 0,20;
| 1 | SIMPLE | table1 | range | section,price | section | 4 |
NULL | 128966 | Using where; Using filesort |
explain select * from table1 where price>1 and section=1 order by
price limit 0,20;
| 1 | SIMPLE | table1 | range | section,price | price | 4
| NULL | 128951 | Using where |
I'm not sure why these 'explains' looks different to the previous
example. For some reason, the second query (price>1) is using the
correct key, but on the first query, the wrong key is being used and
then a filesort has to take place.
Any ideas why this seems to happen, why should the incorrect key be
selected just by changing the value to qualify on?
I have tried a few more queries that are similar on other tables if
you would like them.
I suspected something like this was happening, but your earlier
EXPLAIN showed no indexes were being used at all, so I was a little
stumped. It didn't seem to make sense otherwise.
Try a USE INDEX and see if you can get a similar query plan on both
queries.
Baron
Ahhh... Thanks Baron!
This does indeed fix the problems :-)
mysql> explain select * from adverts use index (price) where price>0 and
section=1 order by price limit 0,20;
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
| 1 | SIMPLE | table1 | range | price | price | 4 |
NULL | 128978 | Using where |
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from adverts use index (price) where price>1 and
section=1 order by price limit 0,20;
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
| 1 | SIMPLE | table1 | range | price | price | 4 |
NULL | 128951 | Using where |
+----+-------------+---------+-------+---------------+-------+---------+------+--------+-------------+
1 row in set (0.00 sec)
select * from table1 use index (price) where price>1 and section=1 order
by price limit 0,20;
........
20 rows in set (0.00 sec)
select * from table1 use index (price) where price>0 and section=1 order
by price limit 0,20;
........
20 rows in set (0.00 sec)
This all looks good to me :-)
So, I'm just guessing the the query optimisation is going a little awry
on me. I'll have to watch for this in the future.
Thanks for all the help guys (Baron, Dan, Jeremy & Michael), it's much
appreciated.
Chris.
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