Re: How often do I need to reindex tables? - Mailing list pgsql-general

Bill and Tom

Best to find out what kind of index you want to create beforehand
If your data is evenly distributed and exhibits High Cardinality (2 entries
for A,B,C...Z) then I would recommend a BTREE Index
If not (low cardinality scenarios such as gender) then create Bitmap Index
I cant speak for postgres but index creation will necessitate you to
schedule time when you can bring DB offline (such as a weekend) as most DB
will not allow a unique index to be created on a table while the table is in
use
Also I find oracle books and online documentation very helpful specifically
http://otn.oracle.com
Books are available from Oracle Press

HTH
Martin
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----- Original Message -----
From: "Bill Moran" <wmoran@collaborativefusion.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: <pgsql-general@postgresql.org>
Sent: Thursday, April 19, 2007 9:33 AM
Subject: Re: [GENERAL] How often do I need to reindex tables?


> In response to Tom Lane <tgl@sss.pgh.pa.us>:
>
>> Bill Moran <wmoran@collaborativefusion.com> writes:
>> > Just an FYI ... I remembered what prompted the cron job.
>>
>> > We were seeing significant performance degradation.  I never did actual
>> > measurements, but it was on the order of "Bill, why is restoring taking
>> > such a long time?" from other systems people.  At the time, I poked
>> > around
>> > and tried some stuff here and there and found that reindex restored
>> > performance.  I didn't look at actual size at that time.
>>
>> A reindex might improve performance for reasons other than bloat --- to
>> wit, that a freshly-built index is in perfect physical order, which
>> tends to get degraded over time by page splits.  How important that is
>> depends on your usage patterns.  If this is what the story is for your
>> situation, then what might fix it (in 8.2) is to create the index with
>> FILLFACTOR 50 or so, so that it's already at the steady state density
>> and won't need many page splits.
>>
>> > Anyway, I'll report back in a few weeks as to what the numbers look
>> > like.
>>
>> Yeah, please for the moment just watch what happens with the default
>> behavior.
>
> Remember this discussion?
>
> To recap, I had scheduled a weekly reindex of this database because I
> was seeing performance issues otherwise.  In order to see if this was
> actually helping, I disabled the redindex job, ran a few timing
> experiments, then scheduled a job to email me the size of the indexes
> in the database on a daily basis.
>
> At this point, I have daily records of index size since March 6th.
>
> The behaviour is like this:  A freshly created index is about 21,000
> pages in size.  Under normal usage, the index size balloons to about
> 38,000 pages immediately after the first backup job is run.  From there
> it grows slowly (but fairly consistently) by about 100 pages each day.
> As of today, it is 44304 pages.
>
> When I first brought up this discussion, the table contained 8068956
> rows.  It now has 7451381, which means it's dropped by 7%
>
> The important part is that I can't reproduce the performance problems
> that I originally thought were the result of this.  It's entirely
> possible that something else was changed since then that actually
> fixed the problem, and that the index bloat was a red herring.
>
> Not sure what (if any) conclusions can be drawn from this.  Is there
> any other data I should gather?  Have I just proved my previous
> rantings about the necessity of reindexing to be wrong?
>
> --
> Bill Moran
> Collaborative Fusion Inc.
> http://people.collaborativefusion.com/~wmoran/
>
> wmoran@collaborativefusion.com
> Phone: 412-422-3463x4023
>
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