Re: TOAST : partial detoasting : only a small part of a toasted value (for pgpointcloud)

=?UTF-8?Q?R=C3=A9mi_Cura?= <remi(dot)cura(at)gmail(dot)com> writes:
> So the pgpointcloud store sometimes very large groups of points into one
> row (TOASTED), something along few kB to few MB. TOAST would be in EXTERNAL
> mode (no compression).
> Sometimes we only want to access a part of this data (one or several blocks
> within the full data).
>> From what I understand, for the moment it requires to fetch all the data
> from disk, then de-toast it, then select only the part of the data we are
> interested in.

> Yet I think it is possible to detoast only a subset of the data (and thus
> fetch only some part of the data), considering that the [doc on toast](
> https://www.postgresql.org/docs/11/storage-toast.html) says
>> " Use of EXTERNAL will make substring operations on wide text and bytea
> columns faster
>> (at the penalty of increased storage space) because these operations are
> optimized
>> to fetch only the required parts of the out-of-line value when it is not
> compressed. "

> So my question is how does it work, how easy would it be to implement for
> pgpointcloud?

See PG_DETOAST_DATUM_SLICE and users of that macro.

regards, tom lane