Requesting some information about the small portion of source code of postgreSQL

Dear PostgresSQL Hackers,
I am working in optimizer module of postgreSQL 9.4.1. I am trying to
return a subplan for a query instead of full plan. For this I need to
return an intermediate plan (or path) from the DP lattice (i.e. from
*RelOptInfo
*standard_join_search() *at* allpaths.c) *instead of the full optimal plan
(which is from the last level of *root->join_rel_level()).*
while doing so I am getting *error :variable not found in subplan target
lists* at function *fix_join_expr_mutator* at *setrefs.c.*

It will be great if you can give me some idea about why this error is
happening, since this error is happening at *fix_join_expr_mutator* function
at *setrefs.c.* Please give me more information about this portion of code.
Also I would like to know for what targetlist stands for.

Thanks,
Srinivas Karthik