Re: Querying foreign table with SP-GiST index results in “ERROR: cache lookup failed for type 0”

On Thu, Jun 20, 2019 at 6:16 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> On Tue, Jun 18, 2019 at 9:22 PM Jeff Janes <jeff.janes@gmail.com> wrote:
> > On Tue, Jun 18, 2019 at 8:02 AM Joao Ferreira <jpgferreira@yahoo.com> wrote:
> >> I'm using PostgreSQL 11.3 and I want to access a table in another Postgres database using postgres_fdw but since
theoriginal table has an SP-GiST index on a geometry column, the query on the foreign table fails with "ERROR: cache
lookupfailed for type 0", even if the table is empty (Step 7). This doesn't happen if the query is done directly to the
remotetable (Steps 3 and 6) or if the remote table doesn't have the index (Steps 3 and 4). 

> > I've verified this on ubuntu 18.04 with 11.3 installed from PGDG apt repo, and attached a single-file reproduction.
>
> I've also verified this on my environment with PG 11.4.
>
> > If I change from sp-gist to just gist, there is no problem.  And if I change to indexing a built-in sp-gist
operatorclass (point rather than geom), there is also no problem. 
>
> Verified.
>
> Will continue investigations.

I found that this error is thrown by index_can_return()
(spgcanreturn()) called from get_relation_info() for the SP-GiST index
on the locations table.  IIUC, the cause of that is: the SP-GiST
config function defined in PostGIS shown below called from
spgGetCache() in spgcanreturn() doesn't work well for the postgres_fdw
case, in which case cfg->prefixType and cfg->leafType are set to
InvalidOid, causing the "ERROR: cache lookup failed for type 0" error
in fillTypeDesc() callled later in spgGetCache() for the
cfg->prefixType, because TypenameGetTypid() returns InvalidOid in that
case since that in postgres_fdw we run the connection with search_path
restricted to pg_catalog while box2df is created in the public schema
in an environment built using a PostGIS default configuration.

PGDLLEXPORT Datum gserialized_spgist_config_2d(PG_FUNCTION_ARGS)
{
    spgConfigOut *cfg = (spgConfigOut *)PG_GETARG_POINTER(1);

--> Oid boxoid = TypenameGetTypid("box2df");
    cfg->prefixType = boxoid;
    cfg->labelType = VOIDOID; /* We don't need node labels. */
    cfg->leafType = boxoid;
    cfg->canReturnData = false;
    cfg->longValuesOK = false;

    PG_RETURN_VOID();
}

I'm a newbie to PostGIS, so maybe I'm missing something, but isn't it
a bit fragile to use TypenameGetTypid() here?  I just thought it would
be better to set boxoid in a more hard-coded way in this function, not
using TypenameGetTypid().

Best regards,
Etsuro Fujita