Fix bug: Precision for Float column which stores timestamp is too small

[piglei]

Float(16) if not precise enough for storing unix timestamp, this may cause various bugs.
See my script for testing:

# -*- coding: utf-8 -*-
import time
from sqlalchemy import create_engine
from sqlalchemy import Column, Float, MetaData, Table

echo = False

engines = (
    ('postgresql', create_engine('postgresql://DB_CONFIG', echo=echo)),
    ('mysql', create_engine('mysql://DB_CONFIG', echo=echo)),
    ('sqlite', create_engine('sqlite://', echo=echo)),
)

meta_data = MetaData()

test = Table(
    'test', meta_data,
    Column('col16', Float(precision=16)),
    Column('col32', Float(precision=32)),
)

for engine_type, engine in engines:
    print 'Engine:', engine_type
    meta_data.drop_all(engine)
    meta_data.create_all(engine)

    now = time.time()
    print 'Now:', now
    engine.execute('insert into test values (%s, %s)' % (now, now))

    for each in engine.execute(test.select()):
        print 'Float(16): %f Float(32): %f' % tuple(each)

    print 

Output:

Engine: postgresql
Now: 1421572566.87
Float(16): 1421570000.000000 Float(32): 1421572566.870000

Engine: mysql
Now: 1421572566.9
Float(16): 1421570000.000000 Float(32): 1421572566.900000

Engine: sqlite
Now: 1421572566.9
Float(16): 1421572566.900000 Float(32): 1421572566.900000

Only sqlite can give the right result.

[binux]

I only tested sqlalchemy with sqlite. Thank you!