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.

Hi Chris,

Chris Hemmings 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 :-)

Did you run both queries multiple times and average the time taken?
Otherwise, it seems likely that in one instance the data was cached, and
in the other it was not. The query_cache being off does not affect
caching in this sense.

Regards,

Jeremy

--
high performance mysql consulting
www.provenscaling.com

Jeremy Cole wrote:
> Hi Chris,
>
> Chris Hemmings 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 :-)
>
> Did you run both queries multiple times and average the time taken?
> Otherwise, it seems likely that in one instance the data was cached,
> and in the other it was not. The query_cache being off does not
> affect caching in this sense.
>
> Regards,
>
> Jeremy
>

Jeremy, Thanks for the swift reply :-)

I have tried the query multiple times... Just to double check, here are
the timings for each, 5 times:

SELECT *
FROM `table1`
WHERE price >0
AND section =1
ORDER BY price
LIMIT 0 , 30

Showing rows 0 - 29 (128,978 total, Query took 0.9605 sec)
Showing rows 0 - 29 (128,978 total, Query took 0.9506 sec)
Showing rows 0 - 29 (128,978 total, Query took 0.9556 sec)
Showing rows 0 - 29 (128,978 total, Query took 0.9614 sec)
Showing rows 0 - 29 (128,978 total, Query took 0.9474 sec)

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)
Showing rows 0 - 29 (128,949 total, Query took 0.0008 sec)
Showing rows 0 - 29 (128,949 total, Query took 0.0009 sec)
Showing rows 0 - 29 (128,949 total, Query took 0.0008 sec)
Showing rows 0 - 29 (128,949 total, Query took 0.0008 sec)

Wierd huh?

Would you like any of the mysqld runtime settings?

Chris.

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 <lists@cheeky.org> 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.
>
>
>
> --
> MySQL General Mailing List
> For list archives: http://lists.mysql.com/mysql
> To unsubscribe: http://lists.mysql.com/mysql?unsub=drbuettner@gmail.com
>
>

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 <lists@cheeky.org> 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.
>>
>>
>>
>> --
>> MySQL General Mailing List
>> For list archives: http://lists.mysql.com/mysql
>> To unsubscribe: http://lists.mysql.com/mysql?unsub=drbuettner@gmail.com
>>
>>
>

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.

Still confused.....

Chris.

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 <lists@cheeky.org> 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.
>>>
>>>
>>>
>>> --
>>> MySQL General Mailing List
>>> For list archives: http://lists.mysql.com/mysql
>>> To unsubscribe:
>>> http://lists.mysql.com/mysql?unsub=drbuettner@gmail.com
>>>
>>>
>>
>
> 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

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 <lists@cheeky.org> 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.
>>>>
>>>>
>>>>
>>>> --
>>>> MySQL General Mailing List
>>>> For list archives: http://lists.mysql.com/mysql
>>>> To unsubscribe:
>>>> http://lists.mysql.com/mysql?unsub=drbuettner@gmail.com
>>>>
>>>>
>>>
>>
>> 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 guys :-)

Chris.

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 <lists@cheeky.org> 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.
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> MySQL General Mailing List
>>>>> For list archives: http://lists.mysql.com/mysql
>>>>> To unsubscribe:
>>>>> http://lists.mysql.com/mysql?unsub=drbuettner@gmail.com
>>>>>
>>>>>
>>>>
>>>
>>> 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 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.

Thanks,

Chris.