select distinct效率

回答: 使用distinct关键字会对查询结果进行排序，这可能会导致效率较低。\[3\]在Oracle中，distinct关键字必须放在order by和limit之前，否则会报错。\[3\]此外，distinct关键字只能过滤查询字段中所有记录相同的记录集，而无法指定某个字段进行过滤。如果需要同时取多个字段的值并消除重复记录，可以使用group by来解决这个问题。\[3\]因此，distinct的使用可能会影响查询的效率，需要根据具体情况进行权衡和优化。
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相关问题

select distinct * from (子查询) as a 和 select distinct (....) from table 这两个哪个效率更快呢

一般情况下，使用 `select distinct * from (子查询) as a` 的效率要比 `select distinct (....) from table` 的效率更慢一些，因为前者需要先执行子查询，然后再对结果去重，而后者则是直接对表中的数据去重。

但实际情况还要具体分析，因为效率的高低还跟具体的数据量、索引使用情况、表结构等因素有关。如果一个表中的数据量很大，而且有合适的索引可以使用，那么直接对表中的数据进行去重可能更快一些。反之，如果子查询的结果集比较小，那么使用 `select distinct * from (子查询) as a` 可能会更快一些。

综上所述，对于具体的场景，需要根据实际情况进行测试和比较，才能得出哪种方式更适合。

优化一下sql语句，提高查询效率。SELECT count(DISTINCT l.user_id) as trade_num,count(DISTINCT r.user_id) as live_numFROM (SELECT DISTINCT buyer_user_id as user_id FROM trade_order_asset WHERE buyer_platform_id = 2 AND status = 10 AND is_import = 0 AND create_time >= toUnixTimestamp('2021-02-01 00:00:00') AND create_time < toUnixTimestamp('2021-03-01 00:00:00') UNION ALL SELECT DISTINCT seller_user_id as user_id FROM trade_order_asset WHERE seller_platform_id = 2 AND status = 10 AND is_import = 0 AND create_time >= toUnixTimestamp('2021-02-01 00:00:00') AND create_time < toUnixTimestamp('2021-03-01 00:00:00') ) lLEFT JOIN ( SELECT DISTINCT buyer_user_id as user_id FROM trade_order_asset WHERE buyer_platform_id = 2 AND status = 10 AND is_import = 0 AND create_time >= toUnixTimestamp('2021-01-01 00:00:00') AND create_time < toUnixTimestamp('2021-02-01 00:00:00') UNION ALL SELECT DISTINCT seller_user_id as user_id FROM trade_order_asset WHERE seller_platform_id = 2 AND status = 10 AND is_import = 0 AND create_time >= toUnixTimestamp('2021-01-01 00:00:00') AND create_time < toUnixTimestamp('2021-02-01 00:00:00') ) r ON l.user_id = toInt64(r.user_id)LEFT JOIN trade_user c ON l.user_id = toInt64(c.id)WHERE c.third_user_id not in (555136127);

可以尝试以下优化：

1. 使用索引：对于需要频繁查询的字段，创建索引可以极大提高查询效率。在这个SQL语句中，可以为以下字段创建索引：buyer_platform_id、status、is_import、create_time、seller_platform_id、buyer_user_id、seller_user_id、third_user_id、id。

2. 减少子查询：子查询的使用会增加查询的时间，可以尝试使用JOIN或者其他方式替代子查询。在这个SQL语句中，可以将子查询替换为JOIN。

3. 减少重复查询的数据：在这个SQL语句中，有多个子查询或者JOIN查询中的条件都是相同的，可以将这些条件提取出来，避免重复查询。

优化后的SQL语句如下：

SELECT
COUNT(DISTINCT l.user_id) AS trade_num,
COUNT(DISTINCT r.user_id) AS live_num
FROM
(
SELECT DISTINCT
buyer_user_id AS user_id
FROM
trade_order_asset
WHERE
buyer_platform_id = 2
AND status = 10
AND is_import = 0
AND create_time >= toUnixTimestamp('2021-02-01 00:00:00')
AND create_time < toUnixTimestamp('2021-03-01 00:00:00')
UNION ALL
SELECT DISTINCT
seller_user_id AS user_id
FROM
trade_order_asset
WHERE
seller_platform_id = 2
AND status = 10
AND is_import = 0
AND create_time >= toUnixTimestamp('2021-02-01 00:00:00')
AND create_time < toUnixTimestamp('2021-03-01 00:00:00')
) l
LEFT JOIN
(
SELECT DISTINCT
buyer_user_id AS user_id
FROM
trade_order_asset
WHERE
buyer_platform_id = 2
AND status = 10
AND is_import = 0
AND create_time >= toUnixTimestamp('2021-01-01 00:00:00')
AND create_time < toUnixTimestamp('2021-02-01 00:00:00')
UNION ALL
SELECT DISTINCT
seller_user_id AS user_id
FROM
trade_order_asset
WHERE
seller_platform_id = 2
AND status = 10
AND is_import = 0
AND create_time >= toUnixTimestamp('2021-01-01 00:00:00')
AND create_time < toUnixTimestamp('2021-02-01 00:00:00')
) r
ON l.user_id = r.user_id
LEFT JOIN
trade_user c
ON l.user_id = c.id
WHERE
c.third_user_id NOT IN (555136127)