理解ActiveRecord触发的sql查询

我有两个表，一个是trip，一个是txns，在oracledb中有一对多关系。 当我发出下面的查询时

Trip.where("upper(trips.domain) = 'FOOBAR.COM'  and txns.user_id = 50001 and txns.txn_type = '1'").includes([:txns]).order('trips.created_at desc').limit(10).offset(0)

为此，sql日志中有两个查询

    Trip Load (4920.4ms)   SELECT * FROM (
    SELECT raw_sql_.*, rownum raw_rnum_
    FROM (SELECT DISTINCT "TM"."TRIPS".id, FIRST_VALUE(trips.created_at)
 OVER (PARTITION BY "TM"."TRIPS".id ORDER BY trips.created_at desc) AS alias_0__ FROM "TM"."TRIPS" LEFT OUTER JOIN "TXNS" ON "TXNS"."TRIP_ID" = "TM"."TRIPS"."ID"
 WHERE (upper(trips.domain) = 'FOOBAR.COM' and txns.txn_type = '1') 
ORDER BY alias_0__ DESC) raw_sql_
    )
    WHERE raw_rnum_ between 1 and 10

    SQL (186.2ms)  SELECT "TM"."TRIPS"."ID" AS t0_r0, "TM"."TRIPS"."TRIP_REF" AS t0_r1, "TM"."TRIPS"."TRIP_NAME" AS t0_r2, 
"TM"."TRIPS"."START_DATE_TIME" AS t0_r3, "TM"."TRIPS"."END_DATE_TIME" AS t0_r4, "TM"."TRIPS"."AMOUNT" AS t0_r5, "TM"."TRIPS"."CREATED_AT" AS t0_r6,
 "TM"."TRIPS"."UPDATED_AT" AS t0_r7, "TM"."TRIPS"."USER_ID" AS t0_r8, 
"TM"."TRIPS"."BOOKING_STATUS" AS t0_r9, "TM"."TRIPS"."TRAVELLERS" AS t0_r10, 
"TM"."TRIPS"."USER_TRIP_NAME" AS t0_r11, "TM"."TRIPS"."CONTACT_DETAIL_ID" AS t0_r12, 
"TM"."TRIPS"."AIR" AS t0_r13, "TM"."TRIPS"."HOTEL" AS t0_r14, "TM"."TRIPS"."DOMAIN" AS t0_r15, "TM"."TRIPS"."TRAIN" AS t0_r16, "TM"."TRIPS"."TAGS" AS t0_r17, "TM"."TRIPS"."CURRENCY" AS t0_r18, "TM"."TRIPS"."CUR_INR_VALUE" AS t0_r19, 
"TM"."TRIPS"."COMPANY_ID" AS t0_r20, "TXNS"."ID" AS t1_r0, "TXNS"."TRIP_ID" AS t1_r1,
 "TXNS"."USER_ID" AS t1_r2, "TXNS"."TXN_TYPE" AS t1_r3, "TXNS"."STATUS" AS t1_r4, "TXNS"."SOURCE_TYPE" AS t1_r5, "TXNS"."SOURCE_ID" AS t1_r6, "TXNS"."EXTERNAL_REFS" AS t1_r7, "TXNS"."CREATED_AT" AS t1_r8, "TXNS"."IP_NUMBER" AS t1_r9, "TXNS"."MISC" AS t1_r10 FROM "TM"."TRIPS" LEFT OUTER JOIN "TXNS" ON "TXNS"."TRIP_ID" = "TM"."TRIPS"."ID" WHERE "TM"."TRIPS"."ID" IN (11620660, 11620651, 11620649, 11620647, 11620646, 11620645, 11620644, 11620642, 11620641, 11620636) AND (upper(trips.domain) = 'FOOBAR.COM' and txns.txn_type = '1') ORDER BY trips.created_at desc

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我想知道为什么active record会引发两个查询

答案可以在关于Rails

include

与Rails

连接之间的差异中看到

得到两个查询的原因是您使用的是基于连接的include（因为您的where子句引用了

txns

关联中的列）因为你有一个极限

因为您要加入一个has-many-rails，所以不能在查询中只保留一个限制10——举一个极端的例子，如果您有一次与10个关联TXN的行程，那么将限制10添加到查询rails通常会运行，那么只会返回该1次行程（与10个匹配TXN中的每一个相连），而不是10次不同的行程

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为了解决这个问题，rails首先查找与您的条件匹配的前10个Trip行，然后启动第二个查询，以加载包含您想要的关联的那些Trip行。

因此在SQL中，我无法执行与parent.limit（10）.join（children）和have limit（children）等效的操作？