[IMPALA-2233] Impala should not choose a function signature that results in loss of precision

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: Impala 2.1, Impala 2.2
• Fix Version/s: Impala 2.3.0
• Labels: correctness
• Target Version: Impala 2.3.0
• Assignee: Tim Armstrong
• Reporter: casey
• Created: 21/Aug/15 06:36
• Updated: 16/Nov/17 01:20
• Resolved: 01/Oct/15 14:43

Description

When arguments to a function require casting because they don't match an exact function signature, the chosen signature should not result in loss of precision of the input arguments. (If loss of precision is unavoidable an error or warning should be given.)

Example of current behavior

Query: select greatest(0, cast('99999.8888' as decimal(30, 10)))
+---------------------------------------------------+
| greatest(0, cast('99999.8888' as decimal(30,10))) |
+---------------------------------------------------+
| 99999.890625                                      |
+---------------------------------------------------+

Query: create table foo as select greatest(0, cast('99999.8888' as decimal(30, 10)))
+-------------------+
| summary           |
+-------------------+
| Inserted 1 row(s) |
+-------------------+
Fetched 1 row(s) in 3.72s

Query: describe foo
+------+-------+---------+
| name | type  | comment |
+------+-------+---------+
| _c0  | float |         |
+------+-------+---------+

Postgres and mysql both preserve the precision

mysql> select greatest(0, cast('99999.8888' as decimal(30, 10)));
+----------------------------------------------------+
| greatest(0, cast('99999.8888' as decimal(30, 10))) |
+----------------------------------------------------+
|                                   99999.8888000000 |
+----------------------------------------------------+

mysql> create table foo as select greatest(0, cast('99999.8888' as decimal(30, 10)));
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> desc foo;
+----------------------------------------------------+----------------+------+-----+--------------+-------+
| Field                                              | Type           | Null | Key | Default      | Extra |
+----------------------------------------------------+----------------+------+-----+--------------+-------+
| greatest(0, cast('99999.8888' as decimal(30, 10))) | decimal(30,10) | NO   |     | 0.0000000000 |       |
+----------------------------------------------------+----------------+------+-----+--------------+-------+

Issue Links

• is duplicated by: IMPALA-1879 mod() function doesn't always interpret fractional arguments as DECIMAL
• relates to: IMPALA-1713 ISNULL(A, B) - Impala should automaticaly cast second argument from type B to type A, if they are different and it possible.