Re: [HACKERS] casts: max double precision text double precision fails with out or range error
Excerpts from Maciej Sakrejda's message of mar ene 11 03:28:13 -0300 2011: Tried asking this in pgsql-general but I got no response, so I thought I'd give hackers a shot: postgres=# select (((1.7976931348623157081e+308)::double precision)::text)::double precision; ERROR: 1.79769313486232e+308 is out of range for type double precision I'm working on a pg driver and in my float data decoder functional tests, I ran into some errors that I eventually traced back to this behavior. Essentially, postgres seems to cast the max normal double (i.e., the bits of ~(1ULL52 | 1ULL63)) to text in such a manner that it's rounded up, and the reverse cast, text-to-double-precision, does not recognize it as being in range. Curiously, pg_dump seems to print doubles with more precision (in both COPY and INSERT modes), avoiding this issue. Yeah, it sets the extra_float_digits parameter. alvherre=# set extra_float_digits to 3; SET alvherre=# select (((1.7976931348623157081e+308)::double precision)::text)::double precision; float8 ── 1.79769313486231571e+308 (1 fila) -- Álvaro Herrera alvhe...@commandprompt.com The PostgreSQL Company - Command Prompt, Inc. PostgreSQL Replication, Consulting, Custom Development, 24x7 support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] casts: max double precision text double precision fails with out or range error
Tried asking this in pgsql-general but I got no response, so I thought I'd give hackers a shot: postgres=# select (((1.7976931348623157081e+308)::double precision)::text)::double precision; ERROR: 1.79769313486232e+308 is out of range for type double precision I'm working on a pg driver and in my float data decoder functional tests, I ran into some errors that I eventually traced back to this behavior. Essentially, postgres seems to cast the max normal double (i.e., the bits of ~(1ULL52 | 1ULL63)) to text in such a manner that it's rounded up, and the reverse cast, text-to-double-precision, does not recognize it as being in range. Curiously, pg_dump seems to print doubles with more precision (in both COPY and INSERT modes), avoiding this issue. Of course I'm not expecting perfect precision in round-tripping doubles like this (this is always dicey with IEEE floating point anyway), but failing outright is a little ugly. Any thoughts? Version is PostgreSQL 8.4.6 on i486-pc-linux-gnu, compiled by GCC gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3, 32-bit. Also, although the simplest way to illustrate this problem is with this round-trip set of casts, that's obviously a contrived use case. However, given that the same behavior is seen in the TEXT mode output for doubles of the FEBE protocol, I think it's a little more noteworthy. Thanks, Maciek Sakrejda -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
