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Re: [HACKERS] Re: ALTER TABLE DROP COLUMN

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jan Wieck <wieck(at)debis(dot)com>
Cc: Don Baccus <dhogaza(at)pacifier(dot)com>, Hannu Krosing <hannu(at)tm(dot)ee>, PostgreSQL Development <pgsql-hackers(at)postgreSQL(dot)org>
Subject: Re: [HACKERS] Re: ALTER TABLE DROP COLUMN
Date: 2000-02-29 05:40:19
Message-ID: 27878.951802819@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
wieck(at)debis(dot)com (Jan Wieck) writes:
>     So  it's  definitely  some kind of "accept duplicates for now
>     but check for final dup's on this key later".

>     But that requires another index scan later. We  can  remember
>     the  relations  and indices Oid (to get back the relation and
>     index   in   question)   plus   the   CTID   of   the   added
>     (inserted/updated   tuple)   to   get  back  the  key  values
>     (remembering the key itself could blow up memory). Then do an
>     index   scan   under   current  (statement  end/XACT  commit)
>     visibility to check if more than one HeapTupleSatisfies().

>     It'll be expensive, compared to current UNIQUE implementation
>     doing  it  on  the fly during btree insert (doesn't it?). But
>     the only way I see.

How about:

1. During INSERT into unique index, notice whether any other index
entries have same key.  If so, add that key value to a queue of
possibly-duplicate keys to check later.

2. At commit, or whenever consistency should be checked, scan the
queue.  For each entry, use the index to look up all the matching
tuples, and check that only one will be valid if the transaction
commits.

This avoids a full index scan in the normal case, although it could
be pretty slow in the update-every-tuple scenario...

			regards, tom lane

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