Analyze plan of foreign data wrapper

[Mathieu PUJOL]

Hello!
I'am writing a foreign data wrapper. To avoid returning data for a column
that is not used, I parse 'targetlist' and 'qual' of the Plan.
I'am able to find Var nodes but I can't figure out how i could now if this
node is related to a column my foreign table.
I guess that the field 'location' is the solution but I don't understand
how to use it to retrieve target table oid and column index.
I hope someone could help me.
Thanks in advance
-- 
Mathieu PUJOL
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Tel : 05.81.33.13.36
REAL FUSIO - 3D Computer Graphics
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Re: Analyze plan of foreign data wrapper

[Mathieu PUJOL]

Hi,
I understand that I should also use varno to check which table is
referenced by varattno. In case of Join, aggregation, etc. Sometimes I get
a number or INNER_VAR or OUTER_VAR.
I am lost on how i could resolve this.
I understand that OUTER_VAR/INNER_VAR are related to joins sub plans. Is
outer related to left plan and inner to right plan ? In this case varattno
is index of target list of subplan ?
When varno is an index how to retrieve table info ?
Regards
Mathieu


Le jeu. 28 juin 2018 à 23:17, Laurenz Albe  a
écrit :

> Mathieu PUJOL wrote:
> > I'am writing a foreign data wrapper. To avoid returning data for a
> column that is not used, I parse 'targetlist' and 'qual' of the Plan.
> > I'am able to find Var nodes but I can't figure out how i could now if
> this node is related to a column my foreign table.
>
> For a Var v, v->varattno contains the attribute number of the column.
> That is the same as the attnum column in pg_attribute.
>
> If v->varattno == 0, it is a whole-row reference, like in
>
>SELECT mytab FROM mytab;
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>

Re: Analyze plan of foreign data wrapper

[Mathieu PUJOL]

I read it many times but I'am not entirely familiar with concepts of range
table and I'am not sure to fully understant all implications.
For now I have a workaround by parsing only plan's target list and by
checking if resorigtbl is equal to oid of my table. The main drawback is
that I can't detect column if it is used in a function or an aggregator.
Thanks anyway for your help.

Le mar. 3 juil. 2018 à 12:38, Laurenz Albe  a
écrit :

> Mathieu PUJOL wrote:
> > I understand that I should also use varno to check which table is
> referenced by varattno. In case of Join, aggregation, etc. Sometimes I get
> a number or INNER_VAR or OUTER_VAR.
> > I am lost on how i could resolve this.
> > I understand that OUTER_VAR/INNER_VAR are related to joins sub plans. Is
> outer related to left plan and inner to right plan ? In this case varattno
> is index of target list of subplan ?
> > When varno is an index how to retrieve table info ?
>
> I have no deep understanding of these things.
>
> Maybe the following comment from include/nodes/primnodes.h can help:
>
> /*
>  * Var - expression node representing a variable (ie, a table column)
>  *
>  * Note: during parsing/planning, varnoold/varoattno are always just copies
>  * of varno/varattno.  At the tail end of planning, Var nodes appearing in
>  * upper-level plan nodes are reassigned to point to the outputs of their
>  * subplans; for example, in a join node varno becomes INNER_VAR or
> OUTER_VAR
>  * and varattno becomes the index of the proper element of that subplan's
>  * target list.  Similarly, INDEX_VAR is used to identify Vars that
> reference
>  * an index column rather than a heap column.  (In ForeignScan and
> CustomScan
>  * plan nodes, INDEX_VAR is abused to signify references to columns of a
>  * custom scan tuple type.)  In all these cases, varnoold/varoattno hold
> the
>  * original values.  The code doesn't really need varnoold/varoattno, but
> they
>  * are very useful for debugging and interpreting completed plans, so we
> keep
>  * them around.
>  */
> #defineINNER_VAR65000   /* reference to inner subplan */
> #defineOUTER_VAR65001   /* reference to outer subplan */
> #defineINDEX_VAR65002   /* reference to index column */
>
> #define IS_SPECIAL_VARNO(varno) ((varno) >= INNER_VAR)
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>