Hi tom, Thanks for the quick respond. Why are SELECT query never marked nullable? For nullable columns, when I call SPI_getvalue(), the result (in char*) is NULL. I don’t think I’m too clear on the definition of attnotnull. Can you give me a example in which the tupleTable is can be marked nullable? Also, is there any other ways to get nullability of each column while getting the data from SPI_cursor_fetch? The only way I can think is to call another separate command to query the table schema, but it will be in a separate transaction in that case.
Thank you again! Best, Ivy > On Aug 17, 2018, at 6:02 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Wu Ivy <ivywu...@gmail.com> writes: >> I’m currently building a Postgres C extension that fetch data from a >> Postgres table. >> Since the table can be large, in order to prevent memory overrun, I use >> SPI_cursor_fetch to fetch chunks of data. The result rows are saved in >> SPITupleTable* SPI_tuptable and attributes are saved in >> SPI_tuptable->tupdesc. >> In order to process my data, I need to get information of column nullability >> (whether column has NOT NULL constrain). I can get this information by >> calling: > >> TupleDesc tupdesc = SPI_tuptable->tupdesc; >> bool is_nullable = TupleDescAttr(tupdesc, column_num - 1) -> attnotnull; >> However, the result (is_nullable) is always 0, meaning the column does not >> have NOT NULLl constraint, even for columns that do have the NOT NULL >> constraint. > > The output columns of a SELECT query are never marked nullable, regardless > of what the source data was. > > regards, tom lane
