Re: [GSoC2014] Patch ALTER TABLE ... SET LOGGED

On Thu, Jul 17, 2014 at 8:02 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
>
> > That means should I "FlushRelationBuffers(rel)" before change the
> > relpersistence?
>
> I don't think that'd help. I think what this means that you simply
> cannot change the relpersistence of the old relation before the heap
> swap is successful. So I guess it has to be something like (pseudocode):
>
> OIDNewHeap = make_new_heap(..);
> newrel = heap_open(OIDNewHeap, AEL);
>
> /*
> * Change the temporary relation to be unlogged/logged. We have to do
> * that here so buffers for the new relfilenode will have the right
> * persistency set while the original filenode's buffers won't get read
> * in with the wrong (i.e. new) persistency setting. Otherwise a
> * rollback after the rewrite would possibly result with buffers for the
> * original filenode having the wrong persistency setting.
> *
> * NB: This relies on swap_relation_files() also swapping the
> * persistency. That wouldn't work for pg_class, but that can't be
> * unlogged anyway.
> */
> AlterTableChangeCatalogToLoggedOrUnlogged(newrel);
> FlushRelationBuffers(newrel);
> /* copy heap data into newrel */
> finish_heap_swap();
>
> And then in swap_relation_files() also copy the persistency.
>
>
> That's the best I can come up right now at least.
>

Isn't better if we can set the relpersistence as an argument to
"make_new_heap" ?

I'm thinking to change the make_new_heap:

From:

make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, bool forcetemp,
LOCKMODE lockmode)

To:

make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, char relpersistence,
LOCKMODE lockmode)

That way we can create the new heap with the appropriate relpersistence, so
in the swap_relation_files also copy the persistency, of course.

And after copy the heap data to the new table (ATRewriteTable) change
relpersistence of the OldHeap's indexes, because in the "finish_heap_swap"
they'll be rebuild.

Thoughts?

Regards,

--
Fabrízio de Royes Mello
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