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Thread: optimizer woes ?

optimizer woes ?

From
Howie
Date:
i've got a table with about 420,000 rows in it and two indexes:

Table    = logins
+----------------------------------+----------------------------------+-------+
|              Field               |              Type                | Length|
+----------------------------------+----------------------------------+-------+
| nick                             | varchar() not null               |    30 |
| channum                          | int4 not null                    |     4 |
| hostname                         | varchar() not null               |   120 |
| dttime                           | datetime not null default now (  |     8 |
| logtype                          | char() not null default 'I'      |     1 |
+----------------------------------+----------------------------------+-------+
Indices:  logins_hostname_idx         logins_nick_idx


logins_hostname_idx is on, oddly enough, logins.hostname
logins_nick_idx is on logins.nick

neither index is unique.

table is VACUUM ANALYZE'd every night.  so, why would this query not be
using the index on logins.nick?

---[ CUT ]---
ircbot=> explain select nick,hostname,dttime,logtype from logins where
nick=lower('anick') ORDER BY dttime desc;
NOTICE:  QUERY PLAN:

Sort  (cost=19913.31 size=0 width=0) ->  Seq Scan on logins  (cost=19913.31 size=42498 width=44)
---[ CUT ]---

but when removing the lower(), it uses the index:

---[ CUT ]---
ircbot=> explain select nick,hostname,dttime,logtype from logins where
nick='anick' ORDER BY dttime desc;

NOTICE:  QUERY PLAN:

Sort  (cost=15.68 size=0 width=0) ->  Index Scan using logins_nick_idx on logins  (cost=15.68 size=214
width=44)
---[ CUT ]---

shouldn't the optimizer convert lower('anick') to lowercase first, then
use that value in searching the logins_nick_idx index ?

btw, this is using postgres 6.4.2 (system is a debian box, p2-350 with
256m, all uw scsi </brag> )

---
Howie <caffeine@toodarkpark.org>   URL: http://www.toodarkpark.org    
"The distance between insanity and genius is measured only by success."

Re: [SQL] optimizer woes ?

From
Bruce Momjian
Date:
> ircbot=> explain select nick,hostname,dttime,logtype from logins where
> nick=lower('anick') ORDER BY dttime desc;
> NOTICE:  QUERY PLAN:
> 
> Sort  (cost=19913.31 size=0 width=0)
>   ->  Seq Scan on logins  (cost=19913.31 size=42498 width=44)
> ---[ CUT ]---
> 
> but when removing the lower(), it uses the index:
> 
> ---[ CUT ]---
> ircbot=> explain select nick,hostname,dttime,logtype from logins where
> nick='anick' ORDER BY dttime desc;
> 
> NOTICE:  QUERY PLAN:
> 
> Sort  (cost=15.68 size=0 width=0)
>   ->  Index Scan using logins_nick_idx on logins  (cost=15.68 size=214
> width=44)
> ---[ CUT ]---
> 
> shouldn't the optimizer convert lower('anick') to lowercase first, then
> use that value in searching the logins_nick_idx index ?
> 
> btw, this is using postgres 6.4.2 (system is a debian box, p2-350 with
> 256m, all uw scsi </brag> )
> 

We have on our TODO list:
* Use index with constants on functions

Seems we have not implemented it yet.  It may be done in 6.5, but I
don't think so.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026

Re: [SQL] optimizer woes ?

From
Tom Lane
Date:
>> shouldn't the optimizer convert lower('anick') to lowercase first, then
>> use that value in searching the logins_nick_idx index ?

The system only knows how to use qualifications like "var rel constant"
as index-scan restrictions.  "var rel func(constant)" is not of that
form ... but it could be if there were a preprocessing step that
recognized "func(constant)" as a constant subexpression and replaced it
by its result.

Right now, we don't have any such step.  I've been thinking about it for
6.6 though.

Bruce Momjian <maillist@candle.pha.pa.us> writes:
> We have on our TODO list:
>     * Use index with constants on functions

The TODO entry is insufficiently ambitious: it should read "implement
a general-purpose constant-subexpression-reduction step".  (Actually,
I think that TODO entry might refer to something completely different
... wasn't the complaint that you couldn't make an index on
"date_part('date', field)"? )
        regards, tom lane

Re: [SQL] optimizer woes ?

From
Bruce Momjian
Date:
> The TODO entry is insufficiently ambitious: it should read "implement
> a general-purpose constant-subexpression-reduction step".  (Actually,

Changed:
* Convert function(constant) into a constant for index use

> I think that TODO entry might refer to something completely different
> ... wasn't the complaint that you couldn't make an index on
> "date_part('date', field)"? )

This is a separate issue.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026