Le mar. 13 nov. 2018 à 19:00, Tom Lane <tgl@sss.pgh.pa.us> a écrit :
Rémi Cura <remi.cura@gmail.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.
