将LIMIT添加到ARRAY_TO_JSON或ARRAY_AGG

时间：2018-01-20 11:51:27

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

在使用PostgreSQL 9.6.6作为后端的双人游戏中，我定义了以下自定义存储函数，用于为用户检索聊天消息：

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;

它加入words_games和words_chat表并生成以下JSON对象（游戏ID为＃34; 9＆＃34;作为字符串键），其中包含带有消息的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)

这很有效，但我想将LIMIT 10添加到数组元素的数量 - 作为对抗聊天泛滥的措施。

我已尝试将其添加到该功能中（请参阅上面的注释行），但它没有效果。

你能为LIMIT 10建议正确的位置吗？

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泛滥聊天来摧毁此类对象的大小。

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;

但遗憾的是得到了语法错误：

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

更新2：

以下是我使用的两个表，抱歉不包括之前的信息 - 

#\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

2 个答案:

答案 0 :(得分：2)

也许this会回答您的问题。尝试做：

(ARRAY_AGG( /* same as before */ ))[1:10] AS y

答案 1 :(得分：1)

你的功能应该是这样的：

CREATE OR REPLACE FUNCTION words_get_user_chat(in_uid integer)
  RETURNS jsonb AS
$func$
   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
$func$ LANGUAGE sql STABLE;

不要聚合所有行，只是为了稍后丢弃剩余。会是一种浪费。将LIMIT放在ORDER BY后面的子查询中。

您需要首先从gid标识符合条件的words_games，然后使用LATERAL加入words_chat上的子查询。也应该是正确和快速的。

由于c.created已定义为NOT NULL，因此您无需在NULLS LAST子句中添加ORDER BY。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);

也许是words_games上的一些索引。取决于基数和价值频率。

在参与其中时，我还简化了jsonb结果的构建。

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