I reviewed the v4 patch, and here are some comments from me.
* After applying this patch, pull_varattnos() should not called in
unnecessary places. For developers who want list of
columns-to-be-processed for some purpose, it would be nice to mention
when they should use pull_varattnos() and when they should not, maybe
at the comments of pull_varattnos() implementation.
* deparseReturningList() and postgresGetForeignRelSize() in
contrib/postgres_fdw/ also call pull_varattnos() to determine which
column to be in the SELECT clause of remote query. Shouldn't these be
replaced in the same manner? Other pull_varattnos() calls are for
restrictions, so IIUC they can't be replaced.
* Through this review I thought up that lazy evaluation approach might
fit attr_needed. I mean that we leave attr_needed for child relations
empty, and fill it at the first request for it. This would avoid
useless computation of attr_needed for child relations, though this
idea has been considered but thrown away before...
2014-08-20 18:55 GMT+09:00 Etsuro Fujita <fujita.ets...@lab.ntt.co.jp>:
> Hi Ashutish,
> (2014/08/14 22:30), Ashutosh Bapat wrote:
>> On Thu, Aug 14, 2014 at 10:05 AM, Etsuro Fujita
>> <fujita.ets...@lab.ntt.co.jp <mailto:fujita.ets...@lab.ntt.co.jp>> wrote:
>> (2014/08/08 18:51), Etsuro Fujita wrote:
>> >>> (2014/06/30 22:48), Tom Lane wrote:
>> >>>> I wonder whether it isn't time to change that. It was coded
>> like that
>> >>>> originally only because calculating the values would've been a
>> waste of
>> >>>> cycles at the time. But this is at least the third place
>> where it'd be
>> >>>> useful to have attr_needed for child rels.
>> > I've revised the patch.
>> There was a problem with the previous patch, which will be described
>> below. Attached is the updated version of the patch addressing that.
>> Here are some more comments
> Thank you for the further review!
>> attr_needed also has the attributes used in the restriction clauses as
>> seen in distribute_qual_to_rels(), so, it looks unnecessary to call
>> pull_varattnos() on the clauses in baserestrictinfo in functions
>> check_selective_binary_conversion(), remove_unused_subquery_outputs(),
> IIUC, I think it's *necessary* to do that in those functions since the
> attributes used in the restriction clauses in baserestrictinfo are not added
> to attr_needed due the following code in distribute_qual_to_rels.
> * If it's a join clause (either naturally, or because delayed by
> * outer-join rules), add vars used in the clause to targetlists of
> * relations, so that they will be emitted by the plan nodes that scan
> * those relations (else they won't be available at the join node!).
> * Note: if the clause gets absorbed into an EquivalenceClass then this
> * may be unnecessary, but for now we have to do it to cover the case
> * where the EC becomes ec_broken and we end up reinserting the original
> * clauses into the plan.
> if (bms_membership(relids) == BMS_MULTIPLE)
> List *vars = pull_var_clause(clause,
> add_vars_to_targetlist(root, vars, relids, false);
>> Although in case of RTE_RELATION, the 0th entry in attr_needed
>> corresponds to FirstLowInvalidHeapAttributeNumber + 1, it's always safer
>> to use it is RelOptInfo::min_attr, in case get_relation_info() wants to
>> change assumption or somewhere down the line some other part of code
>> wants to change attr_needed information. It may be unlikely, that it
>> would change, but it will be better to stick to comments in RelOptInfo
>> 443 AttrNumber min_attr; /* smallest attrno of rel (often
>> <0) */
>> 444 AttrNumber max_attr; /* largest attrno of rel */
>> 445 Relids *attr_needed; /* array indexed [min_attr ..
>> max_attr] */
> Good point! Attached is the revised version of the patch.
> Best regards,
> Etsuro Fujita
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