Re: [HACKERS] Custom compression methods

On 3/19/21 8:25 PM, Tom Lane wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> On 2021-Mar-19, Robert Haas wrote:
>>> Well, I really do hope that some day in the bright future, pglz will
>>> no longer be the thing we're shipping as the postgresql.conf default.
>>> So we'd just be postponing the noise until then. I think we need a
>>> better idea than that.
>> Hmm, why?  In that future, we can just change the pg_dump behavior to no
>> longer dump the compression clause if it's lz4 or whatever better
>> algorithm we choose.  So I think I'm clarifying my proposal to be "dump
>> the compression clause if it's different from the compiled-in default"
>> rather than "different from the GUC default".
> Extrapolating from the way we've dealt with similar issues
> in the past, I think the structure of pg_dump's output ought to be:
>
> 1. SET default_toast_compression = 'source system's value'
> in among the existing passel of SETs at the top.  Doesn't
> matter whether or not that is the compiled-in value.
>
> 2. No mention of compression in any CREATE TABLE command.
>
> 3. For any column having a compression option different from
> the default, emit ALTER TABLE SET ... to set that option after
> the CREATE TABLE.  (You did implement such a SET, I trust.)
>
> This minimizes the chatter for the normal case where all or most
> columns have the same setting, and more importantly it allows the
> dump to be read by older PG systems (or non-PG systems, or newer
> systems built without --with-lz4) that would fail altogether
> if the CREATE TABLE commands contained compression options.
> To use the dump that way, you do have to be willing to ignore
> errors from the SET and the ALTERs ... but that beats the heck
> out of having to manually edit the dump script to get rid of
> embedded COMPRESSION clauses.
>
> I'm not sure whether we'd still need to mess around beyond
> that to make the buildfarm's existing upgrade tests happy.
> But we *must* do this much in any case, because as it stands
> this patch has totally destroyed some major use-cases for
> pg_dump.
>
> There might be scope for a dump option to suppress mention
> of compression altogether (comparable to, eg, --no-tablespaces).
> But I think that's optional.  In any case, we don't want
> to put people in a position where they should have used such
> an option and now they have no good way to recover their
> dump to the system they want to recover to.
>
>             


I'm fairly sure this prescription would satisfy the buildfarm. It sounds
pretty sane to me - I'd independently come to a very similar conclusion
before reading the above.


cheers


andrew


--
Andrew Dunstan
EDB: https://www.enterprisedb.com