Cassandra 2.1：通过嵌套UDT的递归 [英] Cassandra 2.1: Recursion by nesting UDT's

问题描述

我在玩用户定义的类型，发现你可以这样做：

  cqlsh：test> ; CREATE TYPE ping（时间时间戳）; 
 cqlsh：test> CREATE TYPE pong（时间时间戳，ping冷冻< ping>）; 
 cqlsh：test> ALTER TYPE ping ADD pong frozen< pong> ;; 
 
 
 cqlsh：test> DESC类型ping; 
 
 CREATE TYPE test.ping（
 time timestamp，
 pong frozen< pong> 
）; 
 
 cqlsh：test> DESC类型pong; 
 
 CREATE TYPE test.pong（
 time timestamp，
 ping frozen< ping> 
）; 
  

这是否与任何用例相关？

> Java驱动程序票。

该模式似乎是递归的，但是当你实际尝试插入数据时不起作用：

  //（为了清楚起见，使用int而不是时间）
 cqlsh：test>创建类型ping（pingid int）; 
 cqlsh：test>创建类型pong（pongid int，ping frozen< ping>）; 
 cqlsh：test> alter type ping ADD pong frozen< pong> ;; 
 cqlsh：test> create table foo（ping frozen< ping> primary key）; 
 
 //这些都可以：
 
 cqlsh：test> insert into foo（ping）values（{pingid：1}）; 
 
 cqlsh：test>插入到foo（ping）值（
 {pingid：1，
 pong：{pongid：2，
 ping：{pingid：3}}}）; 
 
 //但是请注意，当你嵌套一个级别时会发生什么：
 
 cqlsh：test>插入到foo（ping）值（
 {pingid：1，
 pong：{pongid：2，
 ping：{pingid：3，
 pong：{pongid：4} }}}）; 
 InvalidRequest：code = 2200 [无效的查询] message =未知的字段'pong'在用户的值
定义类型ping
  

看起来像 pong 时使用的 ping defined是一个副本，没有看到ALTER语句的效果。所以我的猜测是递归是不允许的，有一个缺失的检查。

一个有趣的副作用是你既不能删除 ping

code>或 pong 之后： - ）

---

EDIT：这确实是 ALTER TYPE 不允许的。请参阅 CASSANDRA-10339 。

I was playing around with the user-defined types and found out you can do something like this:

cqlsh:test> CREATE TYPE ping(time timestamp);
cqlsh:test> CREATE TYPE pong(time timestamp, ping frozen <ping>);
cqlsh:test> ALTER TYPE ping ADD pong frozen <pong>;


cqlsh:test> DESC TYPE ping ;

CREATE TYPE test.ping (
    time timestamp,
    pong frozen<pong>
);

cqlsh:test> DESC TYPE pong ;

CREATE TYPE test.pong (
    time timestamp,
    ping frozen<ping>
);

Is this relevant for any use case?

解决方案

Just came across this while working on a closely related Java driver ticket.

The schema appears to be recursive, but that doesn't work when you actually try to insert data:

// (using int instead of time for the sake of clarity)
cqlsh:test> create type ping(pingid int);
cqlsh:test> create type pong(pongid int, ping frozen<ping>);
cqlsh:test> alter type ping ADD pong frozen<pong>;
cqlsh:test> create table foo(ping frozen<ping> primary key);

// These are OK:

cqlsh:test> insert into foo(ping) values( {pingid:1} );

cqlsh:test> insert into foo(ping) values(
    { pingid:1, 
      pong: { pongid:2, 
              ping: {pingid: 3}}} );

// But notice what happens when you nest one more level:

cqlsh:test> insert into foo(ping) values(
    { pingid:1, 
      pong: { pongid:2, 
              ping: {pingid: 3, 
                     pong: {pongid: 4}}}} );
InvalidRequest: code=2200 [Invalid query] message="Unknown field 'pong' in value of user
defined type ping"

Looks like the ping that was used at the time pong was defined is a "copy" that didn't see the effect of the ALTER statement. So my guess is that recursivity is not allowed and there is a missing check. I'll update my answer when I get confirmation from the Cassandra developers.

One interesting side-effect is that you can delete neither ping nor pong after that :-)

---

EDIT: this is indeed something that ALTER TYPE should not allow. See CASSANDRA-10339.

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