Question

我正在尝试恢复我的对象的最后一个状态，但是这个子选择不起作用，我已经看到新版本的cassandra db支持聚合操作和子查询。

select * from event_store 
   where event_version = (select max(event_version) from event_store) 
    ALLOW FILTERING;

语法异常：第1:49行输入'select'没有可行的选择（...来自event_version = [（] select ...）的event_store

Answer 1

您可以使用2个单独的查询来执行此操作：

int HASH_UPDATE(HASH_CTX *c, const void *data_, size_t len)
{
    const unsigned char *data = data_;
    unsigned char *p;
    HASH_LONG l;
    size_t n;

    if (len == 0)
        return 1;

    l = (c->Nl + (((HASH_LONG) len) << 3)) & 0xffffffffUL;
    /*
     * 95-05-24 eay Fixed a bug with the overflow handling, thanks to Wei     
     Dai
     * <weidai@eskimo.com> for pointing it out.
     */
    if (l < c->Nl)              /* overflow */
        c->Nh++;
    c->Nh += (HASH_LONG) (len >> 29); /* might cause compiler warning on
                                   * 16-bit */
    c->Nl = l;

    n = c->num;
    if (n != 0) {
        p = (unsigned char *)c->data;

        if (len >= HASH_CBLOCK || len + n >= HASH_CBLOCK) {
            memcpy(p + n, data, HASH_CBLOCK - n);
            HASH_BLOCK_DATA_ORDER(c, p, 1);
            n = HASH_CBLOCK - n;
            data += n;
            len -= n;
            c->num = 0;
            /*
             * We use memset rather than OPENSSL_cleanse() here 
               deliberately.
             * Using OPENSSL_cleanse() here could be a performance issue. 
               It
             * will get properly cleansed on finalisation so this isn't a
             * security problem.
             */
            memset(p, 0, HASH_CBLOCK); /* keep it zeroed */
        } else {
            memcpy(p + n, data, len);
            c->num += (unsigned int)len;
            return 1;
        }
    }

    n = len / HASH_CBLOCK;
    if (n > 0) {
        HASH_BLOCK_DATA_ORDER(c, data, n);
        n *= HASH_CBLOCK;
        data += n;
        len -= n;
    }

    if (len != 0) {
        p = (unsigned char *)c->data;
        c->num = (unsigned int)len;
        memcpy(p, data, len);
    }
    return 1;
}

然后

select max(event_version) from event_store;

我可能有点过时，但看起来没有子选择支持：

停止处理此故障单

https://issues.apache.org/jira/browse/CASSANDRA-8846

有人试图这样做：

https://github.com/jobmthomas/Cassandra-SubQuery

但总的来说，cassandra并不支持。

如何在cassandra db中进行子选择

1 个答案: