Re: [HACKERS] Partition-wise join for join between (declaratively)partitioned tables

On Tue, Sep 12, 2017 at 1:16 PM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:
> On 2017/09/11 21:07, Ashutosh Bapat wrote:
>> On Mon, Sep 11, 2017 at 5:19 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> On Mon, Sep 11, 2017 at 6:45 AM, Ashutosh Bapat
>>> <ashutosh.bapat@enterprisedb.com> wrote:
>>>> So, all partitioned partitions are getting locked correctly. Am I
>>>> missing something?
>>>
>>> That's not a valid test.  In that scenario, you're going to hold all
>>> the locks acquired by the planner, all the locks acquired by the
>>> rewriter, and all the locks acquired by the executor, but when using
>>> prepared queries, it's possible to execute the plan after the planner
>>> and rewriter locks are no longer held.
>>
>> I see the same thing when I use prepare and execute
>
> So I looked at this a bit closely and came to the conclusion that we may
> not need to keep partitioned table RT indexes in the
> (Merge)Append.partitioned_rels after all, as far as execution-time locking
> is concerned.
>
> Consider two cases:
>
> 1. Plan is created and executed in the same transaction
>
> In this case, locks taken on the partitioned tables by the planner will
> suffice.
>
> 2. Plan is executed in a different transaction from the one in which it
>    was created (a cached plan)
>
> In this case, AcquireExecutorLocks will lock all the relations in
> PlannedStmt.rtable, which must include all partitioned tables of all
> partition trees involved in the query.  Of those, it will lock the tables
> whose RT indexes appear in PlannedStmt.nonleafResultRelations with
> RowExclusiveLock mode.  PlannedStmt.nonleafResultRelations is a global
> list of all partitioned table RT indexes obtained by concatenating
> partitioned_rels lists of all ModifyTable nodes involved in the query
> (set_plan_refs does that).  We need to distinguish nonleafResultRelations,
> because we need to take the stronger lock on a given table before any
> weaker one if it happens to appear in the query as a non-result relation
> too, to avoid lock strength upgrade deadlock hazard.
>
> Moreover, because all the tables from plannedstmt->rtable, including the
> partitioned tables, will be added to PlannedStmt.relationsOids, any
> invalidation events affecting the partitioned tables (for example,
> add/remove a partition) will cause the plan involving partitioned tables
> to be recreated.
>
> In none of this do we rely on the partitioned table RT indexes appearing
> in the (Merge)Append node itself.  Maybe, we should just remove
> partitioned_rels from (Merge)AppendPath and (Merge)Append node in a
> separate patch and move on.
>
> Thoughts?

Yes, I did the same analysis (to which I refer in my earlier reply to
you). I too think we should just remove partitioned_rels from Append
paths. But then the question is those are then transferred to
ModifyTable node in create_modifytable_plan() and use it for something
else. What should we do about that code? I don't think we are really
using that list from ModifyTable node as well, so may be we could
remove it from there as well. What do you think? Does that mean
partitioned_rels isn't used at all in the code?

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company


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