On Jan 27, 2013 8:46 AM,
Sat, Jan 26, 2013 at 12:21:02PM +0600, s9gf4ult@gmail.com wrote
According to the documentation, SQLite stores whatever you give it, paying very little heed to the declared type. If you get SQLite to *compare* two numbers, it will at that point *convert* them to doubles in order to carry out the comparison. This is quite separate from the question of what it can store.
CREATE TABLE t1(val); sqlite> insert into t1 values ('24.24242424') ...> ; sqlite> insert into t1 values ('24.24242423') ...> ; sqlite> select * from t1 order by val; 24.24242423 24.24242424 sqlite> select * from t1 order by val desc; 24.24242424 24.24242423 sqlite> select sum(val) from t1; 48.48484847
it seems Sqlite can work with arbitrary percission data, very good ! Persistent must have ability to store Fixed.
It's not correct. SQLlite stores any value, but it will use arithmetic operations only with double presicion:
sqlite> select val from t1; 1 0.000001 0.00000001 0.0000000001 0.000000000001 0.00000000000001 0.0000000000000001 0.000000000000000001 0.00000000000000000001 0.0000000000000000000001
sqlite> select sum(val) from t1; 1.00000101010101
as you see it has 14 degree.
Let's check another well known floating point problem:
sqlilte> create table t2 ('val') sqlite> insert into t2 values ('0.7'); sqlite> update t2 set val = 11*val-7;
t2 should remain a const sqlite> update t2 set val = 11*val-7; -- 4 times sqlite> select val from t2; 0.699999999989597 sqlite> update t2 set val = 11*val-7; -- 10 times mote sqlite> select val from t2; 0.430171514341321
As you see you have errors. So SQLlite doesn't support arbitrary presision values.
As for me Persistent should at least support a Money type and use correct backend-specific type for them, either a native for big integer.
Let me clarify a bit: 1. Persistent will currently allow you to create a `Money` datatype which internally stores as an integer. 2. What Persistent currently lacks is a PersistValue constructor for arbitrary-precision values. As a result, during marshaling, some data will be lost when converting from NUMERIC to Double. 3. The upcoming change we're discussing for Persistent would just be to add such a constructor. We could theoretically provide some extra PersistField instances as well, but that's not really what's being discussed. HTH, Michael