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PL/Python: Convert numeric to Decimal
The old implementation converted PostgreSQL numeric to Python float, which was always considered a shortcoming. Now numeric is converted to the Python Decimal object. Either the external cdecimal module or the standard library decimal module are supported. From: Szymon Guz <mabewlun@gmail.com> From: Ronan Dunklau <rdunklau@gmail.com> Reviewed-by: Steve Singer <steve@ssinger.info>
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5 files changed

+138
-34
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5 files changed

+138
-34
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‎doc/src/sgml/plpython.sgml

Lines changed: 17 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -310,12 +310,23 @@ $$ LANGUAGE plpythonu;
310310

311311
<listitem>
312312
<para>
313-
PostgreSQL <type>real</type>, <type>double</type>,
314-
and <type>numeric</type> are converted to
315-
Python <type>float</type>. Note that for
316-
the <type>numeric</type> this loses information and can lead to
317-
incorrect results. This might be fixed in a future
318-
release.
313+
PostgreSQL <type>real</type> and <type>double</type> are converted to
314+
Python <type>float</type>.
315+
</para>
316+
</listitem>
317+
318+
<listitem>
319+
<para>
320+
PostgreSQL <type>numeric</type> is converted to
321+
Python <type>Decimal</type>. This type is imported from
322+
the <literal>cdecimal</literal> package if that is available.
323+
Otherwise,
324+
<literal>decimal.Decimal</literal> from the standard library will be
325+
used. <literal>cdecimal</literal> is significantly faster
326+
than <literal>decimal</literal>. In Python 3.3,
327+
however, <literal>cdecimal</literal> has been integrated into the
328+
standard library under the name <literal>decimal</literal>, so there is
329+
no longer any difference.
319330
</para>
320331
</listitem>
321332

‎src/pl/plpython/expected/plpython_types.out

Lines changed: 41 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -213,36 +213,69 @@ CONTEXT: PL/Python function "test_type_conversion_int8"
213213
(1 row)
214214

215215
CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
216-
plpy.info(x, type(x))
216+
# print just the class name, not the type, to avoid differences
217+
# between decimal and cdecimal
218+
plpy.info(x, x.__class__.__name__)
217219
return x
218220
$$ LANGUAGE plpythonu;
219-
/* The current implementation converts numeric to float. */
220221
SELECT * FROM test_type_conversion_numeric(100);
221-
INFO: (100.0, <type 'float'>)
222+
INFO: (Decimal('100'), 'Decimal')
222223
CONTEXT: PL/Python function "test_type_conversion_numeric"
223224
test_type_conversion_numeric
224225
------------------------------
225-
100.0
226+
100
226227
(1 row)
227228

228229
SELECT * FROM test_type_conversion_numeric(-100);
229-
INFO: (-100.0, <type 'float'>)
230+
INFO: (Decimal('-100'), 'Decimal')
231+
CONTEXT: PL/Python function "test_type_conversion_numeric"
232+
test_type_conversion_numeric
233+
------------------------------
234+
-100
235+
(1 row)
236+
237+
SELECT * FROM test_type_conversion_numeric(100.0);
238+
INFO: (Decimal('100.0'), 'Decimal')
230239
CONTEXT: PL/Python function "test_type_conversion_numeric"
231240
test_type_conversion_numeric
232241
------------------------------
233-
-100.0
242+
100.0
243+
(1 row)
244+
245+
SELECT * FROM test_type_conversion_numeric(100.00);
246+
INFO: (Decimal('100.00'), 'Decimal')
247+
CONTEXT: PL/Python function "test_type_conversion_numeric"
248+
test_type_conversion_numeric
249+
------------------------------
250+
100.00
234251
(1 row)
235252

236253
SELECT * FROM test_type_conversion_numeric(5000000000.5);
237-
INFO: (5000000000.5, <type 'float'>)
254+
INFO: (Decimal('5000000000.5'), 'Decimal')
238255
CONTEXT: PL/Python function "test_type_conversion_numeric"
239256
test_type_conversion_numeric
240257
------------------------------
241258
5000000000.5
242259
(1 row)
243260

261+
SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
262+
INFO: (Decimal('1234567890.0987654321'), 'Decimal')
263+
CONTEXT: PL/Python function "test_type_conversion_numeric"
264+
test_type_conversion_numeric
265+
------------------------------
266+
1234567890.0987654321
267+
(1 row)
268+
269+
SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
270+
INFO: (Decimal('-1234567890.0987654321'), 'Decimal')
271+
CONTEXT: PL/Python function "test_type_conversion_numeric"
272+
test_type_conversion_numeric
273+
------------------------------
274+
-1234567890.0987654321
275+
(1 row)
276+
244277
SELECT * FROM test_type_conversion_numeric(null);
245-
INFO: (None, <type 'NoneType'>)
278+
INFO: (None, 'NoneType')
246279
CONTEXT: PL/Python function "test_type_conversion_numeric"
247280
test_type_conversion_numeric
248281
------------------------------

‎src/pl/plpython/expected/plpython_types_3.out

Lines changed: 41 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -213,36 +213,69 @@ CONTEXT: PL/Python function "test_type_conversion_int8"
213213
(1 row)
214214

215215
CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
216-
plpy.info(x, type(x))
216+
# print just the class name, not the type, to avoid differences
217+
# between decimal and cdecimal
218+
plpy.info(x, x.__class__.__name__)
217219
return x
218220
$$ LANGUAGE plpython3u;
219-
/* The current implementation converts numeric to float. */
220221
SELECT * FROM test_type_conversion_numeric(100);
221-
INFO: (100.0, <class 'float'>)
222+
INFO: (Decimal('100'), 'Decimal')
222223
CONTEXT: PL/Python function "test_type_conversion_numeric"
223224
test_type_conversion_numeric
224225
------------------------------
225-
100.0
226+
100
226227
(1 row)
227228

228229
SELECT * FROM test_type_conversion_numeric(-100);
229-
INFO: (-100.0, <class 'float'>)
230+
INFO: (Decimal('-100'), 'Decimal')
231+
CONTEXT: PL/Python function "test_type_conversion_numeric"
232+
test_type_conversion_numeric
233+
------------------------------
234+
-100
235+
(1 row)
236+
237+
SELECT * FROM test_type_conversion_numeric(100.0);
238+
INFO: (Decimal('100.0'), 'Decimal')
230239
CONTEXT: PL/Python function "test_type_conversion_numeric"
231240
test_type_conversion_numeric
232241
------------------------------
233-
-100.0
242+
100.0
243+
(1 row)
244+
245+
SELECT * FROM test_type_conversion_numeric(100.00);
246+
INFO: (Decimal('100.00'), 'Decimal')
247+
CONTEXT: PL/Python function "test_type_conversion_numeric"
248+
test_type_conversion_numeric
249+
------------------------------
250+
100.00
234251
(1 row)
235252

236253
SELECT * FROM test_type_conversion_numeric(5000000000.5);
237-
INFO: (5000000000.5, <class 'float'>)
254+
INFO: (Decimal('5000000000.5'), 'Decimal')
238255
CONTEXT: PL/Python function "test_type_conversion_numeric"
239256
test_type_conversion_numeric
240257
------------------------------
241258
5000000000.5
242259
(1 row)
243260

261+
SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
262+
INFO: (Decimal('1234567890.0987654321'), 'Decimal')
263+
CONTEXT: PL/Python function "test_type_conversion_numeric"
264+
test_type_conversion_numeric
265+
------------------------------
266+
1234567890.0987654321
267+
(1 row)
268+
269+
SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
270+
INFO: (Decimal('-1234567890.0987654321'), 'Decimal')
271+
CONTEXT: PL/Python function "test_type_conversion_numeric"
272+
test_type_conversion_numeric
273+
------------------------------
274+
-1234567890.0987654321
275+
(1 row)
276+
244277
SELECT * FROM test_type_conversion_numeric(null);
245-
INFO: (None, <class 'NoneType'>)
278+
INFO: (None, 'NoneType')
246279
CONTEXT: PL/Python function "test_type_conversion_numeric"
247280
test_type_conversion_numeric
248281
------------------------------

‎src/pl/plpython/plpy_typeio.c

Lines changed: 32 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -16,6 +16,7 @@
1616
#include "utils/builtins.h"
1717
#include "utils/lsyscache.h"
1818
#include "utils/memutils.h"
19+
#include "utils/numeric.h"
1920
#include "utils/syscache.h"
2021
#include "utils/typcache.h"
2122

@@ -35,7 +36,7 @@ static void PLy_output_datum_func2(PLyObToDatum *arg, HeapTuple typeTup);
3536
static PyObject *PLyBool_FromBool(PLyDatumToOb *arg, Datum d);
3637
static PyObject *PLyFloat_FromFloat4(PLyDatumToOb *arg, Datum d);
3738
static PyObject *PLyFloat_FromFloat8(PLyDatumToOb *arg, Datum d);
38-
static PyObject *PLyFloat_FromNumeric(PLyDatumToOb *arg, Datum d);
39+
static PyObject *PLyDecimal_FromNumeric(PLyDatumToOb *arg, Datum d);
3940
static PyObject *PLyInt_FromInt16(PLyDatumToOb *arg, Datum d);
4041
static PyObject *PLyInt_FromInt32(PLyDatumToOb *arg, Datum d);
4142
static PyObject *PLyLong_FromInt64(PLyDatumToOb *arg, Datum d);
@@ -450,7 +451,7 @@ PLy_input_datum_func2(PLyDatumToOb *arg, Oid typeOid, HeapTuple typeTup)
450451
arg->func = PLyFloat_FromFloat8;
451452
break;
452453
case NUMERICOID:
453-
arg->func = PLyFloat_FromNumeric;
454+
arg->func = PLyDecimal_FromNumeric;
454455
break;
455456
case INT2OID:
456457
arg->func = PLyInt_FromInt16;
@@ -516,16 +517,37 @@ PLyFloat_FromFloat8(PLyDatumToOb *arg, Datum d)
516517
}
517518

518519
static PyObject *
519-
PLyFloat_FromNumeric(PLyDatumToOb *arg, Datum d)
520+
PLyDecimal_FromNumeric(PLyDatumToOb *arg, Datum d)
520521
{
521-
/*
522-
* Numeric is cast to a PyFloat: This results in a loss of precision Would
523-
* it be better to cast to PyString?
524-
*/
525-
Datum f = DirectFunctionCall1(numeric_float8, d);
526-
double x = DatumGetFloat8(f);
522+
static PyObject *decimal_constructor;
523+
char *str;
524+
PyObject *pyvalue;
525+
526+
/* Try to import cdecimal. If it doesn't exist, fall back to decimal. */
527+
if (!decimal_constructor)
528+
{
529+
PyObject *decimal_module;
530+
531+
decimal_module = PyImport_ImportModule("cdecimal");
532+
if (!decimal_module)
533+
{
534+
PyErr_Clear();
535+
decimal_module = PyImport_ImportModule("decimal");
536+
}
537+
if (!decimal_module)
538+
PLy_elog(ERROR, "could not import a module for Decimal constructor");
539+
540+
decimal_constructor = PyObject_GetAttrString(decimal_module, "Decimal");
541+
if (!decimal_constructor)
542+
PLy_elog(ERROR, "no Decimal attribute in module");
543+
}
544+
545+
str = DatumGetCString(DirectFunctionCall1(numeric_out, d));
546+
pyvalue = PyObject_CallFunction(decimal_constructor, "s", str);
547+
if (!pyvalue)
548+
PLy_elog(ERROR, "conversion from numeric to Decimal failed");
527549

528-
return PyFloat_FromDouble(x);
550+
return pyvalue;
529551
}
530552

531553
static PyObject *

‎src/pl/plpython/sql/plpython_types.sql

Lines changed: 7 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -86,14 +86,19 @@ SELECT * FROM test_type_conversion_int8(null);
8686

8787

8888
CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
89-
plpy.info(x, type(x))
89+
# print just the class name, not the type, to avoid differences
90+
# between decimal and cdecimal
91+
plpy.info(x, x.__class__.__name__)
9092
return x
9193
$$ LANGUAGE plpythonu;
9294

93-
/* The current implementation converts numeric to float. */
9495
SELECT * FROM test_type_conversion_numeric(100);
9596
SELECT * FROM test_type_conversion_numeric(-100);
97+
SELECT * FROM test_type_conversion_numeric(100.0);
98+
SELECT * FROM test_type_conversion_numeric(100.00);
9699
SELECT * FROM test_type_conversion_numeric(5000000000.5);
100+
SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
101+
SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
97102
SELECT * FROM test_type_conversion_numeric(null);
98103

99104

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