On Wed, 2003-02-26 at 14:54, Tom Lane wrote:
> "Christopher Kings-Lynne" <chriskl(at)familyhealth(dot)com(dot)au> writes:
> >> Why would there be any speed advantage?
> > Is it not faster to add it when all the data is there, rather than
> > evaluating it as each row is inserted, like indexes?
> I don't see why. There are good algorithmic reasons why bulk-loading
> an index is faster than retail insertions --- mainly that btree goes
> out of its way to make it so, with a special code path. But I see
> no reason why checking a constraint expression is going to be any
> faster as a post-pass than when done while loading the data. If
> anything, I'd guess it to be slower because you have to re-read the
One reason for delaying constraint checks until after all data is loaded
is that any CHECK constraints against other tables must be hidden in
functions. For example, we cannot say:
CHECK (col1 > othertable.col2 WHERE id = othertable.id).
Since such checks are hidden, I suppose it will not be possible to
arrange the order of loading in pg_dump to ensure that such checks
succeed; therefore it would be better for any check constraint involving
a function to be delayed till after all data is loaded.
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight, UK http://www.lfix.co.uk/oliver
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"These things have I written unto you that believe on
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have eternal life, and that ye may believe on the name
of the Son of God." I John 5:13
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