Re: [HACKERS] Block level parallel vacuum

On Tue, 14 Jan 2020 at 10:06, Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Tue, 14 Jan 2020 at 03:20, Mahendra Singh Thalor <mahi6run(at)gmail(dot)com> wrote:
> >
> > On Fri, 10 Jan 2020 at 15:51, Sergei Kornilov <sk(at)zsrv(dot)org> wrote:
> > >
> > > Hi
> > > Thank you for update! I looked again
> > >
> > > (vacuum_indexes_leader)
> > > + /* Skip the indexes that can be processed by parallel workers */
> > > + if (!skip_index)
> > > + continue;
> > >
> > > Does the variable name skip_index not confuse here? Maybe rename to something like can_parallel?
> > >
> >
> > Again I looked into code and thought that somehow if we can add a
> > boolean flag(can_parallel) in IndexBulkDeleteResult structure to
> > identify that this index is supporting parallel vacuum or not, then it
> > will be easy to skip those indexes and multiple time we will not call
> > skip_parallel_vacuum_index (from vacuum_indexes_leader and
> > parallel_vacuum_index)
> > We can have a linked list of non-parallel supported indexes, then
> > directly we can pass to vacuum_indexes_leader.
> >
> > Ex: let suppose we have 5 indexes into a table. If before launching
> > parallel workers, if we can add boolean flag(can_parallel)
> > IndexBulkDeleteResult structure to identify that this index is
> > supporting parallel vacuum or not.
> > Let index 1, 4 are not supporting parallel vacuum so we already have
> > info in a linked list that 1->4 are not supporting parallel vacuum, so
> > parallel_vacuum_index will process these indexes and rest will be
> > processed by parallel workers. If parallel worker found that
> > can_parallel is false, then it will skip that index.
> >
> > As per my understanding, if we implement this, then we can avoid
> > multiple function calling of skip_parallel_vacuum_index and if there
> > is no index which can't performe parallel vacuum, then we will not
> > call vacuum_indexes_leader as head of list pointing to null. (we can
> > save unnecessary calling of vacuum_indexes_leader)
> >
> > Thoughts?
> >
>
> We skip not only indexes that don't support parallel index vacuum but
> also indexes supporting it depending on vacuum phase. That is, we
> could skip different indexes at different vacuum phase. Therefore with
> your idea, we would need to have at least three linked lists for each
> possible vacuum phase(bulkdelete, conditional cleanup and cleanup), is
> that right?
>
> I think we can check if there are indexes that should be processed by
> the leader process before entering the loop in vacuum_indexes_leader
> by comparing nindexes_parallel_XXX of LVParallelState to the number of
> indexes but I'm not sure it's effective since the number of indexes on
> a table should be small.
>

Hi,

+ /*
+ * Try to initialize the parallel vacuum if requested
+ */
+ if (params->nworkers >= 0 && vacrelstats->useindex)
+ {
+ /*
+ * Since parallel workers cannot access data in temporary tables, we
+ * can't perform parallel vacuum on them.
+ */
+ if (RelationUsesLocalBuffers(onerel))
+ {
+ /*
+ * Give warning only if the user explicitly tries to perform a
+ * parallel vacuum on the temporary table.
+ */
+ if (params->nworkers > 0)
+ ereport(WARNING,
+ (errmsg("disabling parallel option of vacuum
on \"%s\" --- cannot vacuum temporary tables in parallel",

From v45 patch, we moved warning of temporary table into
"params->nworkers >= 0 && vacrelstats->useindex)" check so if table
don't have any index, then we are not giving any warning. I think, we
should give warning for all the temporary tables if parallel degree is
given. (Till v44 patch, we were giving warning for all the temporary
tables(having index and without index))

Thoughts?

--
Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.com