Re: Minimal logical decoding on standbys

From: Andres Freund <andres(at)anarazel(dot)de>
To: "Drouvot, Bertrand" <bertranddrouvot(dot)pg(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Jeff Davis <pgsql(at)j-davis(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>, fabriziomello(at)gmail(dot)com, tushar <tushar(dot)ahuja(at)enterprisedb(dot)com>, Rahila Syed <rahila(dot)syed(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Minimal logical decoding on standbys
Date: 2023-04-07 02:18:30
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TBH, I don't like the state of 0001 much. I'm working on polishing it now.

A lot of the new functions in slot.h don't seem right to me:
- ObsoleteSlotIsInvalid() - isn't an obsolete slot by definition invalid?
- Why does ObsoleteSlotIsInvalid() sometime check invalidated_at and sometimes
- DoNotInvalidateSlot() seems too generic a name for a function exposed to the
outside of slot.c
- TransactionIdIsValidPrecedesOrEquals() shouldn't be defined in slot.h -
also, it's not actually clear what semantics it's trying to have.
- there's no commonality in naming between the functions used to test if a
slot needs to be invalidated (SlotIsFreshEnough() vs

Leaving naming etc aside, most of these don't seem to belong in slot.h, but
should just be in slot.c - there aren't conceivable users from outside slot.c.

Independent of this patch: What's the point of invalidated_at? The only reads
of it are done like
invalidated = (!XLogRecPtrIsInvalid(s->data.invalidated_at) &&
i.e. the actual LSN is not used.

ISTM that we should just have it be a boolean, and that it should be used by
the different kinds of invalidating a slot.


Andres Freund

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