Re: ALTER TABLE on system catalogs

On 2/8/19, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>  [v2 patch]

I poked this around a bit and found that this mechanism only works for
bootstrapped tables, as those are the only ones where we can scribble
on pg_attribute entries directly during bootstrap. As such, with this
patch we cannot perform ALTER TABLE for pg_index or pg_largeobject*
[1]. IMHO, it's not worth it to introduce new notation unless it
offers complete coverage. If we're willing to only solve the problem
for pg_class and pg_attribute, I'd rather mark the table rather than
the columns, because we already have visibility into CATALOG_VARLEN.
(rough example attached)

On 2/14/19, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote:
> That already exists: 'm': Value can be stored compressed inline
>
> I agree that it seems we should be using that for those tables that
> don't have a toast table.  Maybe the genbki stuff could do it
> automatically for the appropriate catalogs.

The docs say:
(Actually, out-of-line storage will still be performed for such
columns, but only as a last resort when there is no other way to make
the row small enough to fit on a page.)

If we allow 'm' as an exception, would that interfere with this? My
demo patch has this just in case:

-            if (att->attstorage != 'p')
+            if (att->attstorage != 'p' &&
+                !(att->attstorage == 'm' && IsCatalogRelation(rel)))
                 has_toastable_attrs = true;

Here's another idea:  During initdb, do "ALTER TABLE ALTER COLUMN xyz
SET STORAGE MAIN;"
In initdb, we already pass "-O" to allow system table mods, so I think
we would have to just make sure this one statement doesn't try to add
a toast table. We could have genbki.pl emit a file with SQL statements
to cover all necessary tables/columns.


[1] https://www.postgresql.org/message-id/20180928190630.crt43sk5zd5p555h%40alvherre.pgsql

-- 
John Naylor                https://www.2ndQuadrant.com/
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