ENH: Store in SQL using double precision

Better test

Style fix

Whats new fix

Author: Artemy Kolchinsky

doc/source/whatsnew/v0.16.0.txt

@@ -202,3 +202,4 @@ Bug Fixes
 - Fixed issue in the ``xlsxwriter`` engine where it added a default 'General' format to cells if no other format wass applied. This prevented other row or column formatting being applied. (:issue:`9167`)
 - Fixes issue with ``index_col=False`` when ``usecols`` is also specified in ``read_csv``. (:issue:`9082`)
 - Bug where ``wide_to_long`` would modify the input stubnames list (:issue:`9204`)
+- Bug in to_sql not storing float64 values using double precision. (:issue:`9009`)

pandas/io/sql.py

@@ -908,7 +908,7 @@ def _sqlalchemy_type(self, col):
 
         col_type = self._get_notnull_col_dtype(col)
 
-        from sqlalchemy.types import (BigInteger, Float, Text, Boolean,
+        from sqlalchemy.types import (BigInteger, Integer, Float, Text, Boolean,
             DateTime, Date, Time)
 
         if col_type == 'datetime64' or col_type == 'datetime':
@@ -923,10 +923,15 @@ def _sqlalchemy_type(self, col):
                           "database.", UserWarning)
             return BigInteger
         elif col_type == 'floating':
-            return Float
+            if col.dtype == 'float32':
+                return Float(precision=23)
+            else:
+                return Float(precision=53)
         elif col_type == 'integer':
-            # TODO: Refine integer size.
-            return BigInteger
+            if col.dtype == 'int32':
+                return Integer
+            else:
+                return BigInteger
         elif col_type == 'boolean':
             return Boolean
         elif col_type == 'date':
@@ -1187,9 +1192,17 @@ def has_table(self, name, schema=None):
     def get_table(self, table_name, schema=None):
         schema = schema or self.meta.schema
         if schema:
-            return self.meta.tables.get('.'.join([schema, table_name]))
+            tbl = self.meta.tables.get('.'.join([schema, table_name]))
         else:
-            return self.meta.tables.get(table_name)
+            tbl = self.meta.tables.get(table_name)
+
+        # Avoid casting double-precision floats into decimals
+        from sqlalchemy import Numeric
+        for column in tbl.columns:
+            if isinstance(column.type, Numeric):
+                column.type.asdecimal = False
+
+        return tbl
 
     def drop_table(self, table_name, schema=None):
         schema = schema or self.meta.schema
@@ -1198,8 +1211,9 @@ def drop_table(self, table_name, schema=None):
             self.get_table(table_name, schema).drop()
             self.meta.clear()
 
-    def _create_sql_schema(self, frame, table_name, keys=None):
-        table = SQLTable(table_name, self, frame=frame, index=False, keys=keys)
+    def _create_sql_schema(self, frame, table_name, keys=None, dtype=None):
+        table = SQLTable(table_name, self, frame=frame, index=False, keys=keys,
+                         dtype=dtype)
         return str(table.sql_schema())
 
 
@@ -1213,7 +1227,7 @@ def _create_sql_schema(self, frame, table_name, keys=None):
         'sqlite': 'TEXT',
     },
     'floating': {
-        'mysql': 'FLOAT',
+        'mysql': 'DOUBLE',
         'sqlite': 'REAL',
     },
     'integer': {
@@ -1520,13 +1534,13 @@ def drop_table(self, name, schema=None):
         drop_sql = "DROP TABLE %s" % name
         self.execute(drop_sql)
 
-    def _create_sql_schema(self, frame, table_name, keys=None):
+    def _create_sql_schema(self, frame, table_name, keys=None, dtype=None):
         table = SQLiteTable(table_name, self, frame=frame, index=False,
-                            keys=keys)
+                            keys=keys, dtype=dtype)
         return str(table.sql_schema())
 
 
-def get_schema(frame, name, flavor='sqlite', keys=None, con=None):
+def get_schema(frame, name, flavor='sqlite', keys=None, con=None, dtype=None):
     """
     Get the SQL db table schema for the given frame.
 
@@ -1545,11 +1559,14 @@ def get_schema(frame, name, flavor='sqlite', keys=None, con=None):
         Using SQLAlchemy makes it possible to use any DB supported by that
         library.
         If a DBAPI2 object, only sqlite3 is supported.
+    dtype : dict of column name to SQL type, default None
+        Optional specifying the datatype for columns. The SQL type should
+        be a SQLAlchemy type, or a string for sqlite3 fallback connection.
 
     """
 
     pandas_sql = pandasSQL_builder(con=con, flavor=flavor)
-    return pandas_sql._create_sql_schema(frame, name, keys=keys)
+    return pandas_sql._create_sql_schema(frame, name, keys=keys, dtype=dtype)
 
 
 # legacy names, with depreciation warnings and copied docs

pandas/io/tests/test_sql.py

@@ -651,6 +651,14 @@ def test_get_schema(self):
                                     con=self.conn)
         self.assertTrue('CREATE' in create_sql)
 
+    def test_get_schema_dtypes(self):
+        float_frame = DataFrame({'a':[1.1,1.2], 'b':[2.1,2.2]})
+        dtype = sqlalchemy.Integer if self.mode == 'sqlalchemy' else 'INTEGER'
+        create_sql = sql.get_schema(float_frame, 'test', 'sqlite',
+                                    con=self.conn, dtype={'b':dtype})
+        self.assertTrue('CREATE' in create_sql)
+        self.assertTrue('INTEGER' in create_sql)
+
     def test_chunksize_read(self):
         df = DataFrame(np.random.randn(22, 5), columns=list('abcde'))
         df.to_sql('test_chunksize', self.conn, index=False)
@@ -1233,7 +1241,6 @@ def test_dtype(self):
         df.to_sql('dtype_test3', self.conn, dtype={'B': sqlalchemy.String(10)})
         meta.reflect()
         sqltype = meta.tables['dtype_test3'].columns['B'].type
-        print(sqltype)
         self.assertTrue(isinstance(sqltype, sqlalchemy.String))
         self.assertEqual(sqltype.length, 10)
 
@@ -1262,6 +1269,36 @@ def test_notnull_dtype(self):
         self.assertTrue(isinstance(col_dict['Int'].type, sqltypes.Integer))
         self.assertTrue(isinstance(col_dict['Float'].type, sqltypes.Float))
 
+    def test_double_precision(self):
+        V = 1.23456789101112131415
+
+        df = DataFrame({'f32':Series([V,], dtype='float32'),
+                        'f64':Series([V,], dtype='float64'),
+                        'f64_as_f32':Series([V,], dtype='float64'),
+                        'i32':Series([5,], dtype='int32'),
+                        'i64':Series([5,], dtype='int64'),
+                        })
+
+        df.to_sql('test_dtypes', self.conn, index=False, if_exists='replace', 
+            dtype={'f64_as_f32':sqlalchemy.Float(precision=23)})
+        res = sql.read_sql_table('test_dtypes', self.conn)
+        
+        # check precision of float64
+        self.assertEqual(np.round(df['f64'].iloc[0],14), 
+                         np.round(res['f64'].iloc[0],14))
+
+        # check sql types
+        meta = sqlalchemy.schema.MetaData(bind=self.conn)
+        meta.reflect()
+        col_dict = meta.tables['test_dtypes'].columns
+        self.assertEqual(str(col_dict['f32'].type), 
+                         str(col_dict['f64_as_f32'].type))
+        self.assertTrue(isinstance(col_dict['f32'].type, sqltypes.Float))
+        self.assertTrue(isinstance(col_dict['f64'].type, sqltypes.Float))
+        self.assertTrue(isinstance(col_dict['i32'].type, sqltypes.Integer))
+        self.assertTrue(isinstance(col_dict['i64'].type, sqltypes.BigInteger))
+
+
 
 class TestSQLiteAlchemy(_TestSQLAlchemy):
     """