Hello. Sorry if this is not the appropriate mailing list, but I understood that I needed to post here first.
Warning: I'm not a native english speaker ;) I'm implementing a Foreign Data Wrapper, and I'm trying to "optimize" it by parsing the required columns and simple "quals" from the foreignscanstate. Let's suppose I have a table defined like this: create foreign table test ( name character varying, value character varying, bigcolumn bytea ) Populated like this: name | value | bigcolumn ----------+--------+------------------------------------------ test2 | 4 | \x72616e646f6d737472696e676f666279746573 test1 | 1 | \x72616e646f6d737472696e676f666279746573 test2 | 2 | \x72616e646f6d737472696e676f666279746573 test2 | 3 | \x72616e646f6d737472696e676f666279746573 (4 lignes) Now, if I query my table like this (a subquery, joined on the outer query), what info should I be able to parse from the PlanState ? select name, (select max(value) from test t2 where t2.name = t1.name) as max from test t1; I don't really know much about postgresql internals, regarding execution plans, but here is what I guessed from what I managed to extract from the plan tree so far: - The outer query is executed once, restricting only the needed columns - The subquery is executed once for each row, with: - all columns from the table are requested in a "target entry" node, even if the query only need the name and value columns. - the value corresponding to the name from the outer query is somehow passed in as an Expr of type T_Param, in the quals field. How can I retrieve the value from the Param struct ? The source does not help me much with what to do regarding the various fields in the struct. Does postgresql really fetch all columns in a subselect, or am I just parsing the tree in a wrong way ? I've tried to look at the oracle-fdw code, but I don't understand what they are doing with params. Thank you ! -- Ronan Dunklau
