It looks like pg_column_size gives you the actual size on disk, ie after
compression.
What looks interesting for you would be byteaoctetlen or the function it
wraps, toast_raw_datum_size. See src/backend/access/heap/tuptoaster.c.
pg_column_size calls toast_datum_size, while byteaoctetlen/textoctetlen
calls toast_raw_datum_size.
On Sat, 11 Feb 2006, Bruce Momjian wrote:
>
> Have you looked at the 8.1.X buildin function pg_column_size()?
>
> ---------------------------------------------------------------------------
>
> Mark Dilger wrote:
> > Hello, could anyone tell me, for a user contributed variable length data type,
> > how can you access the length of the data without pulling the entire thing from
> > disk? Is there a function or macro for this?
> >
> > As a first cut, I tried using the PG_DETOAST_DATUM_SLICE macro, but to no avail.
> > grep'ing through the release source for version 8.1.2, I find very little
> > usage of the PG_GETARG_*_SLICE and PG_DETOAST_DATUM_SLICE macros (and hence
> > little clue how they are intended to be used.) The only files where I find them
> > referenced are:
> >
> > doc/src/sgml/xfunc.sgml
> > src/backend/utils/adt/varlena.c
> > src/include/fmgr.h
> >
> >
> > I am writing a variable length data type and trying to optimize the disk usage
> > in certain functions. There are cases where the return value of the function
> > can be determined from the length of the data and a prefix of the data without
> > fetching the whole data from disk. (The prefix alone is insufficient -- I need
> > to also know the length for the optimization to work.)
> >
> > The first field of the data type is the length, as follows:
> >
> > typedef struct datatype_foo {
> > int32 length;
> > char data[];
> > } datatype_foo;
> >
> > But when I fetch the function arguments using
> >
> > datatype_foo * a = (datatype_foo *)
> > PG_DETOAST_DATUM_SLICE(PG_GETARG_DATUM(0),0,BLCKSZ);
> >
> > the length field is set to the length of the fetched slice, not the length of
> > the data as it exists on disk. Is there some other function that gets the length
> > without pulling more than the first block?
> >
> > Thanks for any insight,
> >
> > --Mark
> >
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>
>
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