Re: Are all unlogged tables in any case truncated after a server-crash?

Hi David,

thx for your comments and your advice on reading docs on "checkpoint".

Of course consistency is most important to any DBMS, and if in doubt
about that, truncate data rows and restore from WAL.
But in this case, where data is never modified after bulk load, I
thought there might be an undocumented feature or workaround, like ...
  - option to set the datafiles of those tables in read-only mode and
record this in the metadata
  - on server-recovery spare these unlogged tables and indexes from
truncating all data rows

Its truly a "nice to have"-thing, but I have learned now, that there is
not feature like that.

Mart

Am 11.11.2021 um 22:10 schrieb David G. Johnston:
> On Thu, Nov 11, 2021 at 11:39 AM <sch8el(at)posteo(dot)de> wrote:
>
> After
> that all unlogged tables remain completely unchanged (no
> DML-/DDL-Statements). Hence all of my huge unlogged, "static"
> tables get
> never "unclean" and should not be truncated after a server crash.
>
>
> The server cannot make this assumption so it truncates unlogged
> relations upon an unclean shutdown/crash because it has no WAL with
> which to ensure a proper restoration.
>
> BTW, if I set all unlogged tables to logged after bulk load, it takes
> additional 1.5 hours, mainly because of re-indexing, I suppose.
>
>
> More likely it is writing the entire table, and all of its indexes, to
> WAL.
>
> I assume
> that a restart of the database after a server crash takes another 1.5
> hours (reading from WAL) until the database is up and running.
>
>
> That would be incorrect.  See "CHECKPOINT".
>
>
> Therefore I am seeking a strategy, to not tagging those tables as
> "unclean" and not truncating all unlogged tables on server restart.
>
>
> There is no middle ground that I am aware of.  Either the contents of
> the table are in WAL ,or they are not.  If not, they can be lost upon
> an unclean shutdown.  For manually initiated shutdowns you do have the
> option to do so cleanly.
>
> This topic (unlogged optimizations) does draw quite a bit of attention
> every year but so far the problem of proving to the system that the
> physical file on disk is a truly accurate representation of the
> post-crash relation is yet unsolved.
>
> David J.
>