Re: Why is the query not using the index for sorting? - Mailing list pgsql-performance

Many thanks.
I'll give it a try and see what happens.

-----Original Message-----
From: Craig Ringer [mailto:craig@postnewspapers.com.au]
Sent: Sunday, November 22, 2009 3:25 PM
To: Jonathan Blitz
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Why is the query not using the index for sorting?

On 22/11/2009 8:50 PM, Jonathan Blitz wrote:
> I have a table with a number of columns.
>
> I perform
>
> Select *
> from table
> order by a,b
>
> There is an index on a,b which is clustered (as well as indexes on a
> and b alone).
> I have issued the cluster and anyalze commands.
>
> Nevertheless, PostgreSQL performs a Sequential Scan on the table and
> then performs a sort.

PostgreSQL's query planner probably thinks it'll be faster to read the pages
off the disk sequentially then sort them in memory. To use an index instead,
Pg would have to read the whole index from disk
(sequentially) then fetch all the pages off the disk in a probably
near-random order. So it'd be doing more disk I/O, and much more of it would
be random I/O, which is a LOT slower.

So Pg does it the fast way, reading the table into memory then sorting it
there.

The most important thing to understand is that sometimes, a sequential scan
is just the fastest way to do the job.

I suspect you're working on the assumption that Pg can get all the data it
needs from the index, so it doesn't need to read the tables proper.
In some other database systems this *might* be possible if you had an index
on fields "a" and "b" and issued a "select a,b from table" instead of a
"select *". PostgreSQL, though, can not do this. PostgreSQL's indexes do not
contain all the information required to return values from queries, only
enough information to find the places in the main tables where those values
are to be found.

If you want to know more and understand why that's the case, search for the
phrase "covered index" and the words "index visibility". Suffice it to say
that there are pretty good reasons why it works how it does, and there would
be very large downsides to changing how it works as well as large technical
problems to solve to even make it possible. It's to do with the trade-off
between update/insert/delete speeds and query speeds, the cost of "fatter"
indexes taking longer to read from disk, and lots more.

By the way, if you want to test out different query plans for a query to see
which way is faster, you can use the "enable_" parameters like
"enable_seqscan", "enable_hashjoin" etc to control how PostgreSQL performs
queries. There's *LOTS* to be learned about this in the mailing list
archives. You should also read the following page:

 http://www.postgresql.org/docs/current/static/runtime-config-query.html

but understand that the planner method configuration parameters are intended
mostly for testing and performance analysis, not for production use.

If you find a query that's lots faster with a particular enable_ parameter
set to "off", try increasing your statistics targets on the tables / columns
of interest, re-ANALYZEing, and re-testing. See these pages re statistics:

http://www.postgresql.org/docs/current/static/using-explain.html
http://www.postgresql.org/docs/current/static/planner-stats.html
http://www.postgresql.org/docs/current/static/planner-stats-details.html

If after increasing your stats targets the planner still picks a vastly
slower plan, consider posting to the mailing list with the full output of
"EXPLAIN ANALYZE SELECT myquery....", the full exact text of your query, and
your table schema as shown by "\d tablename" in psql. Someone may be able to
help you or at least explain why it's happening.

--
Craig Ringer
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