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On Jan 27, 2013 8:46 AM, <<a href="mailto:alexander.vershilov@gmail.com" target="_blank">alexander.vershilov@gmail.com</a>> wrote:<br>
><br>
> Sat, Jan 26, 2013 at 12:21:02PM +0600, <a href="mailto:s9gf4ult@gmail.com" target="_blank">s9gf4ult@gmail.com</a> wrote<br>
> > > According to the documentation, SQLite stores whatever you give it,<br>
> > > paying very little heed to the declared type. If you get SQLite to<br>
> > > *compare* two numbers, it will at that point *convert* them to doubles<br>
> > > in order to carry out the comparison. This is quite separate from the<br>
> > > question of what it can store.<br>
> ><br>
> > CREATE TABLE t1(val);<br>
> > sqlite> insert into t1 values ('<a href="tel:24.24242424" value="+12424242424" target="_blank">24.24242424</a>')<br>
> > ...> ;<br>
> > sqlite> insert into t1 values ('<a href="tel:24.24242423" value="+12424242423" target="_blank">24.24242423</a>')<br>
> > ...> ;<br>
> > sqlite> select * from t1 order by val;<br>
> > <a href="tel:24.24242423" value="+12424242423" target="_blank">24.24242423</a><br>
> > <a href="tel:24.24242424" value="+12424242424" target="_blank">24.24242424</a><br>
> > sqlite> select * from t1 order by val desc;<br>
> > <a href="tel:24.24242424" value="+12424242424" target="_blank">24.24242424</a><br>
> > <a href="tel:24.24242423" value="+12424242423" target="_blank">24.24242423</a><br>
> > sqlite> select sum(val) from t1;<br>
> > <a href="tel:48.48484847" value="+14848484847" target="_blank">48.48484847</a><br>
> ><br>
> > it seems Sqlite can work with arbitrary percission data, very good !<br>
> > Persistent must have ability to store Fixed.<br>
> ><br>
><br>
> It's not correct. SQLlite stores any value, but it will use arithmetic<br>
> operations only with double presicion:<br>
><br>
> sqlite> select val from t1;<br>
> 1<br>
> 0.000001<br>
> 0.00000001<br>
> 0.0000000001<br>
> 0.000000000001<br>
> 0.00000000000001<br>
> 0.0000000000000001<br>
> 0.000000000000000001<br>
> 0.00000000000000000001<br>
> 0.0000000000000000000001<br>
><br>
> sqlite> select sum(val) from t1;<br>
> 1.00000101010101<br>
><br>
> as you see it has 14 degree.<br>
><br>
> Let's check another well known floating point problem:<br>
><br>
> sqlilte> create table t2 ('val')<br>
> sqlite> insert into t2 values ('0.7');<br>
> sqlite> update t2 set val = 11*val-7;<br>
><br>
> t2 should remain a const<br>
> sqlite> update t2 set val = 11*val-7; -- 4 times<br>
> sqlite> select val from t2;<br>
> 0.699999999989597<br>
> sqlite> update t2 set val = 11*val-7; -- 10 times mote<br>
> sqlite> select val from t2;<br>
> 0.430171514341321<br>
><br>
> As you see you have errors. So SQLlite doesn't support arbitrary<br>
> presision values.<br>
><br>
> As for me Persistent should at least support a Money type and use<br>
> correct backend-specific type for them, either a native for big integer.</p>
<p style>Let me clarify a bit:</p><p style>1. Persistent will currently allow you to create a `Money` datatype which internally stores as an integer.</p><p style>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.</p>
<p style>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.</p>
<p style>HTH,</p><p style>Michael</p>
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