| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | nconway(at)klamath(dot)dyndns(dot)org (Neil Conway) |
| Cc: | PostgreSQL Patches <pgsql-patches(at)postgresql(dot)org> |
| Subject: | Re: UNIQUE predicate |
| Date: | 2002-07-06 21:32:53 |
| Message-ID: | 4791.1025991173@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-patches |
nconway(at)klamath(dot)dyndns(dot)org (Neil Conway) writes:
> The attached patch implements the SQL92 UNIQUE predicate.
The implementation seems to be well short of usefulness in a production
setting, for two reasons: (1) you're accumulating all the tuples into
memory --- what if they don't fit? (2) the comparison step is O(N^2),
which renders the first point rather moot ... a test case large enough
to risk memory exhaustion will not complete in your lifetime.
I think a useful implementation will require work in the planner to
convert the UNIQUE predicate into a SORT/UNIQUE plan structure (somewhat
like the way DISTINCT is implemented, but we just want a boolean
result).
regards, tom lane
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