sql - 将 LIMIT 添加到 ARRAY_TO_JSON 或 ARRAY_AGG

标签 sql postgresql postgresql-9.6 postgresql-json array-agg

在使用 PostgreSQL 9.6.6 作为后端的 2 人游戏中，我定义了以下自定义存储函数来检索用户的聊天消息:

CREATE OR REPLACE FUNCTION words_get_user_chat(
                in_uid integer
        ) RETURNS jsonb AS
$func$
        SELECT COALESCE(
                JSONB_OBJECT_AGG(gid, ARRAY_TO_JSON(y)),
                '{}'::jsonb
        ) FROM (
                SELECT  c.gid,
                        ARRAY_AGG(
                                JSON_BUILD_OBJECT(
                                        'created', EXTRACT(EPOCH FROM c.created)::int,
                                        'uid',     c.uid,
                                        'msg',     c.msg
                                )
                                ORDER BY c.created ASC
                        ) AS y
                FROM      words_chat c
                LEFT JOIN words_games g
                USING     (gid)
                WHERE     in_uid in (g.player1, g.player2)
                AND       (g.finished IS NULL OR g.finished > CURRENT_TIMESTAMP - INTERVAL '1 day')
                GROUP BY  c.gid
                /* LIMIT 10 */
        ) AS x;

$func$ LANGUAGE sql STABLE;

它连接words_games和words_chat表并生成以下JSON对象(游戏ID“9”作为字符串键)保存带有消息的JSON数组:

 # select words_get_user_chat(6);



                                                                                                                   words_get_user_chat                        




--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------
 {"9": [{"msg": "test2", "uid": 6, "created": 1516445342}, {"msg": "test3", "uid": 6, "created": 1516445358}, {"msg": "test4", "uid": 6, "created": 1516445369
}, {"msg": "test5", "uid": 6, "created": 1516445381}, {"msg": "test6", "uid": 6, "created": 1516445405}, {"msg": "test7", "uid": 6, "created": 1516445415}, {"
msg": "test8", "uid": 6, "created": 1516445508}, {"msg": "test9", "uid": 6, "created": 1516445539}, {"msg": "test10", "uid": 6, "created": 1516445743}, {"msg"
: "test11", "uid": 6, "created": 1516445989}, {"msg": "test12", "uid": 6, "created": 1516446101}, {"msg": "test13", "uid": 6, "created": 1516446125}, {"msg": 
"test14", "uid": 6, "created": 1516446145}, {"msg": "test15", "uid": 6, "created": 1516446227}, {"msg": "test16", "uid": 6, "created": 1516446652}, {"msg": "t
est17", "uid": 6, "created": 1516446999}, {"msg": "test18", "uid": 6, "created": 1516447168}, {"msg": "test19", "uid": 6, "created": 1516447229}, {"msg": "tes
t20", "uid": 6, "created": 1516447493}, {"msg": "test21", "uid": 6, "created": 1516447532}, {"msg": "test22", "uid": 6, "created": 1516447555}, {"msg": "test2
3", "uid": 6, "created": 1516448017}, {"msg": "test24", "uid": 6, "created": 1516448062}]}
(1 row)

这效果很好，但我想将 LIMIT 10 添加到数组元素的数量 - 作为防止聊天泛滥的措施。

我尝试将其添加到函数中(请参阅上面的注释行)，但没有效果。

您能否建议LIMIT 10的正确地点？

我通过 WebSockets 将 JSON 对象发送到 Android 应用程序，并希望防止恶意用户通过大量聊天来扩大此类对象的大小。

更新:

我正在尝试迈克的建议:

CREATE OR REPLACE FUNCTION words_get_user_chat(
                in_uid integer
        ) RETURNS jsonb AS
$func$
        SELECT COALESCE(
                JSONB_OBJECT_AGG(gid, ARRAY_TO_JSON(y)),
                '{}'::jsonb
        ) FROM (
                SELECT  c.gid,
                        ROW_NUMBER() OVER (PARTITION BY c.gid) AS rn,
                        ARRAY_AGG(
                                JSON_BUILD_OBJECT(
                                        'created', EXTRACT(EPOCH FROM c.created)::int,
                                        'uid',     c.uid,
                                        'msg',     c.msg
                                )
                                ORDER BY c.created ASC
                        ) AS y
                FROM      words_chat c
                LEFT JOIN words_games g
                USING     (gid)
                WHERE     in_uid in (g.player1, g.player2)
                AND       (g.finished IS NULL OR g.finished > CURRENT_TIMESTAMP - INTERVAL '1 day')
                AND       rn < 10
                GROUP BY  c.gid
        ) AS x;

$func$ LANGUAGE sql STABLE;

但不幸的是得到语法错误:

ERROR:  42703: column "rn" does not exist
LINE 24:                 AND       rn < 10
                                   ^
LOCATION:  errorMissingColumn, parse_relation.c:3194

更新2:

这是我使用的两个表，很抱歉之前没有包含该信息 -

#\d words_chat
                                   Table "public.words_chat"
 Column  |           Type           |                        Modifiers                         
---------+--------------------------+----------------------------------------------------------
 cid     | bigint                   | not null default nextval('words_chat_cid_seq'::regclass)
 created | timestamp with time zone | not null
 gid     | integer                  | not null
 uid     | integer                  | not null
 msg     | text                     | not null
Indexes:
    "words_chat_pkey" PRIMARY KEY, btree (cid)
Foreign-key constraints:
    "words_chat_gid_fkey" FOREIGN KEY (gid) REFERENCES words_games(gid) ON DELETE CASCADE
    "words_chat_uid_fkey" FOREIGN KEY (uid) REFERENCES words_users(uid) ON DELETE CASCADE

# \d words_games
                                   Table "public.words_games"
  Column  |           Type           |                         Modifiers                         
----------+--------------------------+-----------------------------------------------------------
 gid      | integer                  | not null default nextval('words_games_gid_seq'::regclass)
 created  | timestamp with time zone | not null
 finished | timestamp with time zone | 
 player1  | integer                  | not null
 player2  | integer                  | 
 played1  | timestamp with time zone | 
 played2  | timestamp with time zone | 
 state1   | text                     | 
 state2   | text                     | 
 hint1    | text                     | 
 hint2    | text                     | 
 score1   | integer                  | not null
 score2   | integer                  | not null
 hand1    | character(1)[]           | not null
 hand2    | character(1)[]           | not null
 pile     | character(1)[]           | not null
 letters  | character(1)[]           | not null
 values   | integer[]                | not null
 bid      | integer                  | not null
Indexes:
    "words_games_pkey" PRIMARY KEY, btree (gid)
Check constraints:
    "words_games_check" CHECK (player1 <> player2)
    "words_games_score1_check" CHECK (score1 >= 0)
    "words_games_score2_check" CHECK (score2 >= 0)
Foreign-key constraints:
    "words_games_bid_fkey" FOREIGN KEY (bid) REFERENCES words_boards(bid) ON DELETE CASCADE
    "words_games_player1_fkey" FOREIGN KEY (player1) REFERENCES words_users(uid) ON DELETE CASCADE
    "words_games_player2_fkey" FOREIGN KEY (player2) REFERENCES words_users(uid) ON DELETE CASCADE
Referenced by:
    TABLE "words_chat" CONSTRAINT "words_chat_gid_fkey" FOREIGN KEY (gid) REFERENCES words_games(gid) ON DELETE CASCADE
    TABLE "words_moves" CONSTRAINT "words_moves_gid_fkey" FOREIGN KEY (gid) REFERENCES words_games(gid) ON DELETE CASCADE
    TABLE "words_scores" CONSTRAINT "words_scores_gid_fkey" FOREIGN KEY (gid) REFERENCES words_games(gid) ON DELETE CASCADE

最佳答案

你的函数应该像这样工作:

CREATE OR REPLACE FUNCTION words_get_user_chat(in_uid integer)
  RETURNS jsonb AS
  LANGUAGE sql STABLE
$func$
SELECT COALESCE(jsonb_object_agg(gid, y), '{}')
FROM  (
   SELECT gid, jsonb_agg((SELECT j FROM (SELECT created, uid, msg) j)) AS y
   FROM  (
      SELECT DISTINCT gid  -- DISTINCT may be redundant
      FROM   words_games
      WHERE (finished IS NULL
          OR finished > (CURRENT_TIMESTAMP - INTERVAL '1 day'))
      AND    in_uid IN (player1, player2)
      ) g
   CROSS JOIN LATERAL (
      SELECT EXTRACT(EPOCH FROM created)::int AS created
           , uid
           , msg
      FROM   words_chat c
      WHERE  c.gid = g.gid
      ORDER  BY c.created DESC
      LIMIT  10                        --  HERE !!
      ) c 
   GROUP  BY 1
   ) x
$func$;

不要聚合所有行，只是为了稍后丢弃多余的行。会很浪费。将 LIMIT 放置在子查询中的 ORDER BY 后面。

为此，您需要首先从 words_games 中识别合格的 gid，然后使用 LATERAL 连接到 words_chat< 上的子查询。应该是正确的并且更快。

由于c.created定义为NOT NULL，因此不需要在ORDER BY<中添加NULLS LAST/ 子句。这种匹配的多列索引应该会产生最佳的读取性能:

CREATE INDEX ON words_chat(gid, created DESC);

也许还有一些关于 words_games 的索引。取决于基数和值频率。

同时，我还简化了 jsonb 结果的构建。

相关:

关于sql - 将 LIMIT 添加到 ARRAY_TO_JSON 或 ARRAY_AGG，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48355778/

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