Add explicit error when MERGE command is used with hypertables

[iroussos]

What problem do we want to solve?

As discussed in #4929, the new MERGE command (introduced in PG 15) fails with an error when used with hypertables.

Currently a generic catch-all error is presented which is not only confusing, but straightforward wrong (it will be thrown even if the extension is available and preloaded):

ERROR:  invalid INSERT on the root table of hypertable "conditions"
HINT:  Make sure the TimescaleDB extension has been preloaded.

Before adding full support for MERGE (which at the moment we don't know how much effort it will require), we want to make sure that we present a properly descriptive error to TimescaleDB users.

ERROR:  The MERGE command does not support hypertables in this version 
HINT:  Check https://github.com/timescale/timescaledb/issues/4929 for more information and current status

[iroussos]

@horzsolt assigning to you to decide who should work on this as part of https://github.com/timescale/timescaledb-private/issues/972

[iroussos]

The question is how easy is it to identify when MERGE command is used with hypertables? Seems pretty straight forward, but could this delay our plans for PG15 support?

[iroussos]

@charislam can you help us with some feedback on the error message we should have in this case?

We want to inform TimescaleDB users that the newly introduced MERGE command does not work with hypertables right now (2.9.0) (but it works for normal tables).

The idea is to inform them on use of the command (for that small 99%+ that doesn't check release announcements :-) ). My quick proposal:

ERROR:  The MERGE command does not support hypertables in this version 
HINT:  Check https://github.com/timescale/timescaledb/issues/4929 for more information and current status

Seems weird to link to a github issue in there. Maybe we should skip the hint (?)

PS. We'll most probably add support for it around 2.10.0

FYI @nenadmalic (and any ideas on wording)

[charislam]

Hi @iroussos text looks fine to me, no suggestions there. I actually like the hint, it feels more responsive than just saying "nope, this doesn't work". As it's a temp measure I think linking to a GitHub issue is fine

[theblinkingusb]

Thanks for this one - just came across the error and having the github issue there was helpful. Upgraded and now g2g.