Hi, On 2016/03/25 23:49, Onder Kalaci wrote: > Hi hackers, > > As it's documented in the source code, systable_beginscan() could be used > to on non-system tables as well. My question is that, is it possible to > write a C code with systable_beginscan(), systable_getnext() and ScanKeys > which is equivalent to the following query: (Assume that the qual_column is > a text column) > > SELECT id FROM table WHERE qual_column::int = 15; > > In other words, can we cast a column type to another type via these > low-level APIs? Are there any examples in the Postgres source?
Don't think the API advertises any support for that or at least I couldn't find any. However, I am not sure if it's outright impossible to achieve that with some prep - <uglyhack> initialize the scan key (sk_func) with FmgrInfo of a specialized function, that performs the cast before comparing. Such a function would be written to work with fixed pair of source and target types to be able to invoke, say, textout() -> int4in(), before comparing the value with the argument (sk_argument). The argument would have to be of the target type, needless to say </uglyhack>. Most likely, this would be an unrecommended course of action in the long run, so you would be better off considering other ways (like SPI). Thanks, Amit -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
