pgpool-II does not cache some queries even though it should - postgresql

Question

I have a PostgreSQL server running on 10.6 with an openSUSE distribution of Linux, and I just set up pgpool-II, to allow me to cache queries. It works mostly fine, but for unknown reasons, sometimes I get this warning message :
WARNING: memcache: adding table oid maps, failed to create directory:"/var/log/pgpool/oiddir". error:"No such file or directory"
I already created the directory, changed the owner to the user that runs the pgpool server, and allowed read, write, and execution on this directory to the same user.
This message appears when a query is not yet cached, and it doesn't seem to have any impact, i.e. the query is cached as it should and if I do it again, the result is directly pulled from the cache.
But I also have another problem, and I don't know if it's related to the first one :
When I write big queries (using JOINs, subqueries, lots of conditions, etc), pgpool-II will not cache their results (or cache it but not use it, I have no idea), even though the results are not big (less than 500 rows). Also, in this case, I don't get the "oid" warning message. I tried raising the different limits of shared memory pgpool-II is allowed to use (see documentation) but it changed nothing as I expected, because when pgpool-II fails to cache a query due to the lack of available shared memory, it's supposed to return a message like this one :
LOG: pid 13756: pool_add_temp_query_cache: data size exceeds memqcache_maxcache. current:4095 requested:111 memq_maxcache:4096
But in my case, I don't get any message. Examples of both cases below.
1st problem
Simple query, result is cached, "oid" error :
SELECT *
FROM some_table
WARNING: memcache: adding table oid maps, failed to create directory:"/var/log/pgpool/oiddir". error:"No such file or directory"
-- Doing it a second time will just give me the cached result without any warning, as expected
2nd problem
Complex query, result is not cached (or cached and not used), no warning/error message :
SELECT A.geom, A.id, to_char(C.timestamp, 'DD/MM/YY') as date, C.timestamp::time as time, ROUND(C.value) as value
FROM segments A, lines B, ( SELECT DISTINCT ON (B.id) B.id, A.timestamp, ROUND(A.nb1+A.nb2+A.nb3) as value
FROM records A
CROSS JOIN LATERAL (SELECT *
FROM points B
WHERE A.id = B.id
AND A.direction = B.direction
ORDER BY A.position <-> B.geom
LIMIT 1) AS B
ORDER BY B.id, A.timestamp DESC) AS C
WHERE A.id = B.id
AND B.id = C.id
AND A.direction = B.direction
AND B.direction = C.direction
-- Doing it a second time will just directly request the PostgreSQL server again, instead of pulling the result from the cache, as it should if the result was cached.

Answer 1

Your first issue looks like a permissions issue on the oiddir. Basically pgpool can't create the necessary directories for caching. Depending on how you start pgpool (through pgpoolAdmin or command line), you need to
Create the oiddir folder, mkdir /var/log/pgpool/oiddir
Give permissions of the folder to pgpool
Assuming pgpoolAdmin starts pgpool:
chown -R _www /var/log/pgpool/oiddir (I use pgpool on mac os, so the apache user is "_www", change accordingly)
Assuming you start pgpool from command line:
chown -R postgres /var/log/pgpool/oiddir (or you can use your current user instead of postgres)
Your second issue looks based on the first issue. Basically you can't cache because the oiddir isn't created. Fixing the first should allow you to cache.