Re: COPY FREEZE and setting PD_ALL_VISIBLE/visibility map bits - Mailing list pgsql-hackers

Thanks. These patches seem to resolve the TOAST table issue, freezing it 
as expected. I think the code duplication is not an issue, but I wonder 
why heap_insert uses this condition:

     /*
      * ...
      *
      * No need to update the visibilitymap if it had all_frozen bit set
      * before this insertion.
      */
     if (all_frozen_set && ((vmstatus & VISIBILITYMAP_ALL_FROZEN) == 0))

while heap_multi_insert only does this:

     if (all_frozen_set) { ... }

I haven't looked at the details, but shouldn't both do the same thing?

I've done some benchmarks, comparing master and patched version on a 
bunch of combinations (logged/unlogged, no before-insert trigger, 
trigger filtering everything/nothing). On master, the results are:

     group                         copy       vacuum
     -----------------------------------------------
     logged / no trigger           4672          162
     logged / trigger (all)        4914          162
     logged / trigger (none)       1219           11
     unlogged / no trigger         4132          156
     unlogged / trigger (all)      4292          156
     unlogged / trigger (none)     1275           11

and patched looks like this:

     group                         copy       vacuum
     -----------------------------------------------
     logged / no trigger           4669           12
     logged / trigger (all)        4874           12
     logged / trigger (none)       1233           11
     unlogged / no trigger         4054           11
     unlogged / trigger (all)      4185           12
     unlogged / trigger (none)     1222           10

This looks pretty nice - there are no regressions, just speedups in the 
vacuum step. The SQL script used is attached.

However, I've also repeated the test counting all-frozen pages in both 
the main table and TOAST table, and I get this:


master
======

select count(*) from pg_visibility((select reltoastrelid from pg_class 
where relname = 't'));

  count
--------
  100000
(1 row)


select count(*) from pg_visibility((select reltoastrelid from pg_class 
where relname = 't')) where not all_visible;

  count
--------
  100000
(1 row)


patched
=======

select count(*) from pg_visibility((select reltoastrelid from pg_class 
where relname = 't'));

  count
--------
  100002
(1 row)


select count(*) from pg_visibility((select reltoastrelid from pg_class 
where relname = 't')) where not all_visible;

  count
--------
       0
(1 row)

That is - all TOAST pages are frozen (as expected, which is good). But 
now there are 100002 pages, not just 100000 pages. That is, we're now 
creating 2 extra pages, for some reason. I recall Pavan reported similar 
issue with every 32768-th page not being properly filled, but I'm not 
sure if that's the same issue.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company