On Fri, Nov 04, 2005 at 04:13:29PM +0100, Martijn van Oosterhout wrote:
> On Fri, Nov 04, 2005 at 08:38:38AM -0500, mark@mark.mielke.cc wrote:
> > On Thu, Nov 03, 2005 at 09:17:43PM -0500, Tom Lane wrote:
> > > Actually, the real reason we use UTF-8 and not any of the
> > > sorta-fixed-size representations of Unicode is that the backend is by
> > > and large an ASCII, null-terminated-string engine. *All* of the
> > > supported backend encodings are ASCII-superset codes. Making
> > > everything null-safe in order to allow use of UCS2 or UCS4 would be
> > > a huge amount of work, and the benefit is at best questionable.
> > Perhaps on a side note - my intuition (which sometimes lies) would tell
> > me that, if the above is true, the backend is doing unnecessary copies
> > of read-only data, if only, to insert a '\0' at the end of the strings.
> > Is this true?
> It's not quite that bad. Obviously for all on disk datatype zeros are
> allowed. Bit strings, arrays, timestamps, numerics can all have
> embedded nulls and they have a length header.
Are you and Tom conflicting in opinion? :-)
I read "the backend is by and large an ASCII, null-terminated-string
engine" with "we use UTF-8 [for varlena strings?]" as, a lot of the
code assumes varlena strings are '\0' terminated, and an assumption
on my part, that the varlena strings are not stored in the backend
with a '\0' terminator, therefore, they require being copied out,
terminated with a '\0', before they can be used?
Or perhaps I'm just confused. :-)
> > I'm thinking along the lines of the other threads that speak of PostgreSQL
> > being CPU or I/O bound, not disk bound, for many sorts of operations. Is
> > PostgreSQL unnecessary copying string data around (and other data, I would
> > assume).
> Well, there is a bit of copying around while creating tuples and such,
> but it's not to add null terminators.
How much effort (past discussions that I've missed from a decade ago?
hehe) has been put into determining whether a zero-copy architecture,
or really, a minimum copy architecture, would address some of these
bottlenecks? Am I dreaming? :-)
Cheers,
mark
--
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