Re: Odd system-column handling in postgres_fdw join pushdown patch - Mailing list pgsql-hackers

On 2016/03/22 21:10, Ashutosh Bapat wrote:

On Tue, Mar 22, 2016 at 5:05 PM, Etsuro Fujita
<fujita.etsuro@lab.ntt.co.jp <mailto:fujita.etsuro@lab.ntt.co.jp>> wrote:
    On 2016/03/22 14:54, Ashutosh Bapat wrote:
        Earlier in this mail chain, I suggested that the core should
        take care
        of storing the values for these columns. But instead, I think, core
        should provide functions which can be used by FDWs, if they want, to
        return values for those columns. Something like Datum
        get_syscol_value(RelOptInfo/Relation, attno). The function will
        return
        Datum 0 for most of the columns and table's OID for tableoid. My
        0.02.

    What I had in mind was (1) create_foreignscan_plan would create
    Lists from the ForeignScan's fdw_scan_tlist: (a) indexes/OID values
    of tableoids in fdw_scan_tlist, and (b) indexes of xids and cids in
    fdw_scan_tlist, and then (2) ForeignNext would set the OID values
    for the tableoid columns in the scan tuple, using the Lists (a), and
    appropriate values (0 or something) for the xid and cid columns in
    the scan tuple, using the List (b).

Looks Ok to me, although, that way an FDW looses an ability to use its
own values for those columns, in case it wants to. For example, while
using postgres_fdw for sharding, it might help saving xmax, xmin, cmax,
cmin from the foreign server and use them while communicating with the
foreign server.

Yeah, it might be the case.

On second thoughts, I changed my mind; I think it'd be better for the FDW's to set values for tableoids, xids, and cids in the scan tuple. The reason other than your suggestion is because expressions in fdw_scan_tlist that contain such columns are not necessarily simple Vars and because such expressions might be evaluated more efficiently by the FDW than core.  We assume in postgres_fdw that expressions in fdw_scan_tlist are always simple Vars, though.

I'm not sure it's worth providing functions you suggested, because we can't assume that columns in the scan tuple are always simple Var columns, as I said above.