Re: Performance of count(*)

Tom Lane wrote:
> "Craig A. James" <cjames@modgraph-usa.com> writes:
>> Steve Atkins wrote:
>>> As long as you're ordering by some row in the table then you can do that in
>>> straight SQL.
>>>
>>> select a, b, ts from foo where (stuff) and foo > X order by foo limit 10
>>>
>>> Then, record the last value of foo you read, and plug it in as X the next
>>> time around.
>
>> We've been over this before in this forum: It doesn't work as advertised.
>> Look for postings by me regarding the fact that there is no way to tell
>> the optimizer the cost of executing a function.  There's one, for example,
>> on Oct 18, 2006.
>
> You mean
> http://archives.postgresql.org/pgsql-performance/2006-10/msg00283.php
> ?  I don't see anything there that bears on Steve's suggestion.
> (The complaint is obsolete as of CVS HEAD anyway.)

Mea culpa, it's October 8, not October 18:

   http://archives.postgresql.org/pgsql-performance/2006-10/msg00143.php

The relevant part is this:

"My example, discussed previously in this forum, is a classic.  I have a VERY expensive function (it's in the class of
NP-completeproblems, so there is no faster way to do it).  There is no circumstance when my function should be used as
afilter, and no circumstance when it should be done before a join.  But PG has no way of knowing the cost of a
function,and so the optimizer assigns the same cost to every function.  Big disaster. 

"The result?  I can't use my function in any WHERE clause that involves any other conditions or joins.  Only by itself.
PG will occasionally decide to use my function as a filter instead of doing the join or the other WHERE conditions
first,and I'm dead. 

"The interesting thing is that PG works pretty well for me on big tables -- it does the join first, then applies my
expensivefunctions.  But with a SMALL (like 50K rows) table, it applies my function first, then does the join.  A
searchthat completes in 1 second on a 5,000,000 row database can take a minute or more on a 50,000 row database." 

Craig