Dealing with CLUSTER failures

Christopher Kings-Lynne wrote:
> > I don't think that's a bug. You may not intend ever to cluster on that
> > index again, and if you try it will tell you about the problem.
>
> Except it breaks the 'cluster everything' case:
>
> test=# cluster;
> ERROR: cannot cluster when index access method does not handle null values
> HINT: You may be able to work around this by marking column "a" NOT NULL.

I looked over this item, originally posted as:

http://archives.postgresql.org/pgsql-hackers/2005-03/msg01055.php

It seems that if you use an index method that doesn't support NULLs, you
can use ALTER to set the column as NOT NULL, then CLUSTER, and then set
it to allow NULLs, and when you CLUSTER all tables, the cluster errors
out on that table.

I thought about removing the cluster bit when you do the alter or
something like that, but it seems too confusing and error-prone to be
sure we get every case.

I think the main problem is that while cluster is a performance-only
feature, we error out if we can't cluster one table, basically treating
it as though it needs transaction semantics. It doesn't.

This patch throws an ERROR of you cluster a specific index that can't be
clustered, but issues only a WARNING if you are clustering all tables.
This allows it to report the failed cluster but keep going. I also
modified the code to print the index name in case of failure, because
without that the user doesn't know the failing index name in a
database-wide cluster failure.

Here is an example:

test=> cluster test_gist_idx on test;
ERROR: cannot cluster on index "test_gist_idx" because access method
does not handle null values
HINT: You may be able to work around this by marking column "a" NOT NULL.
test=> cluster;
WARNING: cannot cluster on index "test_gist_idx" because access method
does not handle null values
HINT: You may be able to work around this by marking column "a" NOT NULL.
CLUSTER

You can see the ERROR for a specific index, and WARNING for full
database cluster.

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Bruce Momjian | http://candle.pha.pa.us
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