如何获得最后 N 个连接边

Question

我正在编写一个大型聊天应用程序，这是我的数据库架构：

CREATE CLASS User EXTENDS V;
CREATE PROPERTY User.name STRING;

CREATE CLASS Message EXTENDS V;
CREATE PROPERTY Message.text STRING;
CREATE PROPERTY Message.createdAt DATETIME;
CREATE INDEX Message.createdAt ON Message(createdAt) NOTUNIQUE;

CREATE CLASS Send EXTENDS E;

我正在使用轻量级边，我有 200,000 个边连接到 #12:0，如下所示：

CREATE VERTEX User SET name = 'john';
/* #12:0 */

CREATE VERTEX Message SET content = 'Lorem ipsum dolor sit amet', createdAt = SYSDATE();
/* #20:0 */

CREATE EDGE Send FROM #12:0 TO #20:0

我想获取连接到 #12:0 的最后 5 条消息。我尝试了这些查询：

查询 1:

SELECT FROM (
  SELECT EXPAND(OUT('Send')) FROM #12:0
) ORDER BY createdAt DESC LIMIT 5

用了~2s

"explain" 结果：

{
    "result": [
        {
            "@type": "d",
            "@version": 0,
            "documentReads": 200000,
            "current": "#19:66661",
            "recordReads": 200000,
            "fetchingFromTargetElapsed": 377,
            "expandElapsed": 0,
            "orderByElapsed": 5,
            "evaluated": 200000,
            "elapsed": 2416.7283,
            "resultType": "collection",
            "resultSize": 5,
            "@fieldTypes": "documentReads=l,current=x,recordReads=l,fetchingFromTargetElapsed=l,expandElapsed=l,orderByElapsed=l,evaluated=l,elapsed=f"
        }
    ],
    "notification": "Query executed in 2.427 sec. Returned 1 record(s)"
}

查询 2:

SELECT OUT('Send')[199994-199999] FROM #12:0

用了 ~6s

"explain" 结果：

{
    "result": [
        {
            "@type": "d",
            "@version": 0,
            "documentReads": 1,
            "current": "#12:0",
            "recordReads": 1,
            "optimizationElapsed": 0,
            "fetchingFromTargetElapsed": 8749,
            "evaluated": 1,
            "elapsed": 8749.445,
            "resultType": "collection",
            "resultSize": 1,
            "@fieldTypes": "documentReads=l,current=x,recordReads=l,optimizationElapsed=l,fetchingFromTargetElapsed=l,evaluated=l,elapsed=f"
        }
    ],
    "notification": "Query executed in 8.759 sec. Returned 1 record(s)"
}

有没有更快的方法来做到这一点？

请不要参考chat use case...

我正在使用 orientdb 2.2.7

Answer 1

终于！我找到了一种快速的方法。

我已将数据库架构更改为：

CREATE CLASS User EXTENDS V;
CREATE PROPERTY User.name STRING;

CREATE CLASS Message;
CREATE PROPERTY Message.text STRING;
CREATE PROPERTY Message.writer LINK User;
CREATE PROPERTY Message.createdAt DATETIME;
CREATE INDEX Message.writer ON Message(writer) NOTUNIQUE_HASH_INDEX;
CREATE INDEX Message.createdAt ON Message(createdAt) NOTUNIQUE;

我正在使用 Message.writer 连接到 User 而不是使用边缘。

查询：

SELECT FROM Message
WHERE createdAt < sysdate() AND writer = #12:0
ORDER BY createdAt DESC
SKIP 10 LIMIT 5

150 万条记录需要大约 30 毫秒！

重要提示： 注意 WHERE 子句中的 createdAt < sysdate()，你必须在 ORDER BY 字段上写一个虚拟条件，这使得 ORDER BY 使用 createdAt 索引（快 ~700ms）

"explain" 结果（createdAt < sysdate()）：

{
    "result": [
        {
            "@type": "d",
            "@version": 0,
            "documentReads": 252,
            "fullySortedByIndex": true,
            "documentAnalyzedCompatibleClass": 252,
            "recordReads": 252,
            "fetchingFromTargetElapsed": 6,
            "indexIsUsedInOrderBy": true,
            "compositeIndexUsed": 1,
            "current": "#49:1493348",
            "involvedIndexes": [
                "Message.createdAt"
            ],
            "limit": 5,
            "evaluated": 252,
            "elapsed": 6.447579,
            "resultType": "collection",
            "resultSize": 5,
            "@fieldTypes": "documentReads=l,documentAnalyzedCompatibleClass=l,recordReads=l,fetchingFromTargetElapsed=l,compositeIndexUsed=l,current=x,involvedIndexes=e,evaluated=l,user=x,elapsed=f"
        }
    ],
    "notification": "Query executed in 0.032 sec. Returned 1 record(s)"
}

"explain" 结果（没有 createdAt < sysdate()）：

{
    "result": [
        {
            "@type": "d",
            "@version": 0,
            "documentReads": 48512,
            "fullySortedByIndex": false,
            "documentAnalyzedCompatibleClass": 48512,
            "recordReads": 48512,
            "fetchingFromTargetElapsed": 801,
            "indexIsUsedInOrderBy": false,
            "compositeIndexUsed": 1,
            "current": "#49:1499971",
            "involvedIndexes": [
                "Message.writer"
            ],
            "limit": 5,
            "orderByElapsed": 49,
            "evaluated": 48512,
            "elapsed": 853.48004,
            "resultType": "collection",
            "resultSize": 5,
            "@fieldTypes": "documentReads=l,documentAnalyzedCompatibleClass=l,recordReads=l,fetchingFromTargetElapsed=l,compositeIndexUsed=l,current=x,involvedIndexes=e,orderByElapsed=l,evaluated=l,user=x,elapsed=f"
        }
    ],
    "notification": "Query executed in 0.864 sec. Returned 1 record(s)"
}

如何获得最后 N 个连接边

How to get last N connected edges

orientdb

orientdb2.2