On 05/28/2013 04:41 PM, Szymon Guz wrote:
Hi,
I've got a patch.
This is for a plpython enhancement.
There is an item at the TODO list
http://wiki.postgresql.org/wiki/Todo#Server-Side_Languages
"Fix loss of information during conversion of numeric type to Python
float"
This patch uses a decimal.Decimal type from Python standard library
for the plpthon function numeric argument instead of float.
Patch contains changes in code, documentation and tests.
Most probably there is something wrong, as this is my first Postgres
patch :)
Thanks for contributing.
This patch applies cleanly against master and compiles with warnings
plpy_main.c: In function ‘PLy_init_interp’:
plpy_main.c:157:2: warning: ISO C90 forbids mixed declarations and code
[-Wdeclaration-after-statement]
plpy_main.c:161:2: warning: ISO C90 forbids mixed declarations and code
[-Wdeclaration-after-statement]
You can avoid this by moving the declaration of decimal and decimal_dict
to be at the top of the function where mainmod is declared.
Also in this function you've introduced places where it returns with an
error (the PLy_elog(ERROR...) calls before decrementing the reference to
mainmod. I think you can decrement the mainmod reference after the call
to SetItemString before your changes that import the Decimal module.
The patch works as expected, I am able to write python functions that
take numerics as arguments and work with them. I can adjust the decimal
context precision inside of my function.
One concern I have is that this patch makes pl/python functions
involving numerics more than 3 times as slow as before.
create temp table b(a numeric);
insert into b select generate_series(1,10000);
create or replace function x(a numeric,b numeric) returns numeric as $$
if a==None:
return b
return a+b
$$ language plpythonu;
create aggregate sm(basetype=numeric, sfunc=x,stype=numeric);
test=# select sm(a) from b;
sm
----------
50005000
(1 row)
Time: 565.650 ms
versus before the patch this was taking in the range of 80ms.
Would it be faster to call numeric_send instead of numeric_out and then
convert the sequence of Int16's to a tuple of digits that can be passed
into the Decimal constructor? I think this is worth trying and testing,
Documentation
=================
Your patched version of the docs say
PostgreSQL <type>real</type>, <type>double</type>, and
<type>numeric</type> are converted to
Python <type>Decimal</type>. This type is imported
from<literal>decimal.Decimal</literal>.
I don't think this is correct, as far as I can tell your not changing
the behaviour for postgresql real and double types, they continue to use
floating point.
<listitem>
<para>
PostgreSQL <type>real</type> and <type>double</type>are converted to
Python <type>float</type>.
</para>
</listitem>
<listitem>
<para>
PostgreSQL <type>numeric</type> is converted to
Python <type>Decimal</type>. This type is imported from
<literal>decimal.Decimal</literal>.
</para>
</listitem>
Maybe?
Steve
thanks,
Szymon
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