From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
Cc: | Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Reducing the overhead of NUMERIC data |
Date: | 2005-11-03 15:32:03 |
Message-ID: | 22263.1131031923@sss.pgh.pa.us |
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Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
>> On Thu, 3 Nov 2005, Simon Riggs wrote:
>>> At the moment we've established we can do this fairly much for free.
> Agreed. With the proposal, we are saving perhaps 5% storage space for
> numeric fields, but are adding code complexity and reducing its possible
> precision.
Having to invent UNKNOWNNUMERIC is hardly what I'd call "for free".
That takes it out of the realm of being a small localized project.
I'd feel a lot happier about this if we could keep the dynamic range
up to, say, 10^512 so that it's still true that NUMERIC can be a
universal parse-time representation. That would also make it even
more unlikely that anyone would complain about loss of functionality.
To do that we'd need 8 bits for weight (-128..127 for a base-10K
exponent is enough) but we need 9 bits for dscale which does not
quite fit. I think we could make it go by cramming the sign and
the high-order dscale bit into the first NumericDigit --- the
digit itself can only be 0..9999 so there are a couple of bits
to spare. This probably *would* slow down packing and unpacking of
numerics, but just by a couple lines of C. Arguably the net reduction
in I/O costs would justify that.
regards, tom lane
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