cassandra 存储二进制data

2021-10-09 21:24:38

Blob type

The Cassandra blob data type represents a constant hexadecimal number defined as 0[xX](hex)+ where hex is a hexadecimal character, such as [0-9a-fA-F]. For example, 0xcafe. The maximum theoretical size for a blob is 2 GB. The practical limit on blob size, however, is less than 1 MB. A blob type is suitable for storing a small image or short string.

Blob conversion functions

These functions convert the native types into binary data (blob):

typeAsBlob(value)
blobAsType(value)

For every native, nonblob data type supported by CQL, the typeAsBlob function takes a argument of that data type and returns it as a blob. Conversely, the blobAsType function takes a 64-bit blob argument and converts it to a value of the specified data type, if possible.

This example shows how to use bigintAsBlob:

CREATE TABLE bios ( user_name varchar PRIMARY KEY, 
   bio blob
 );

INSERT INTO bios (user_name, bio) VALUES ('fred', bigintAsBlob(3));
 
SELECT * FROM bios;
 
 user_name | bio
-----------+--------------------
      fred | 0x0000000000000003

This example shows how to use blobAsBigInt.

ALTER TABLE bios ADD id bigint;

INSERT INTO bios (user_name, id) VALUES ('fred', blobAsBigint(0x0000000000000003));

SELECT * FROM bios;

 user_name | bio                | id
-----------+--------------------+----
      fred | 0x0000000000000003 |  3

官方说了，见 https://datastax.github.io/python-driver/getting_started.html?highlight=blob ：

Type Conversions

For non-prepared statements, Python types are cast to CQL literals in the following way:

Python Type	CQL Literal Type
`None`	`NULL`
`bool`	`boolean`
`float`	`float` `double`
`int` `long`	`int` `bigint` `varint` `smallint` `tinyint` `counter`
`decimal.Decimal`	`decimal`
`str` `unicode`	`ascii` `varchar` `text`
`buffer` `bytearray`	`blob`

摘自：https://github.com/datastax/python-driver/blob/master/tests/integration/standard/test_types.py

    def test_can_insert_blob_type_as_bytearray(self):
        """
        Tests that blob type in Cassandra maps to bytearray in Python
        """
        s = self.session

        s.execute("CREATE TABLE blobbytes (a ascii PRIMARY KEY, b blob)")

        params = ['key1', bytearray(b'blob1')]
        s.execute("INSERT INTO blobbytes (a, b) VALUES (%s, %s)", params)

        results = s.execute("SELECT * FROM blobbytes")[0]
        for expected, actual in zip(params, results):
            self.assertEqual(expected, actual)

最后的可以工作的python代码：

from cassandra.cluster import Cluster

cluster = Cluster(["10.178.209.161"])
session = cluster.connect('my_keyspace')

s = session
s.execute("CREATE TABLE blobbytes (a ascii PRIMARY KEY, b blob)")
params = ['key1', bytearray(b'blob1')]
s.execute("INSERT INTO blobbytes (a, b) VALUES (%s, %s)", params)
results = s.execute("SELECT * FROM blobbytes")[0]
for expected, actual in zip(params, results):
     print (expected, actual)

本文转自张昺华-sky博客园博客，原文链接：http://www.cnblogs.com/bonelee/p/6515854.html，如需转载请自行联系原作者

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Blob type

Blob conversion functions

Type Conversions

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