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.
Any further comments on this? I still think it's a bug, ie. an invariant that's not being maintained...
Chris
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