Re: How well clustered is a table?

In response to Jonathan Blitz :
> I was wondering if there is some indication of how well clustered a table is.
>
> In other words, when a Cluster command is performed then a table would be 100%
> clustered.
> As updates etc are made the table clowly loses its clustering.
> Is there any indication as to how "bad" it is at any one point?

Sure, because PG needs this information. See system_view pg_stats, column
correlation. This ranges from -1 to +1. When the value is near -1 or +1,
an index scan on the column will be estimated to be cheaper than when it
is near zero, due to reduction of random access to the disk.

For instance:

test=# select correlation from pg_stats where tablename='foo' and attname='key';
correlation
-------------
0.112908
(1 Zeile)

Zeit: 1,409 ms
test=*# cluster foo using idx_key;
CLUSTER
Zeit: 728,885 ms
test=*# analyse foo;
ANALYZE
Zeit: 59,970 ms
test=*# select correlation from pg_stats where tablename='foo' and attname='key';
correlation
-------------
1
(1 Zeile)

Zeit: 1,505 ms

Andreas
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Andreas Kretschmer
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