oracle中一段sql查询优化的思考,第一段sql是原来的,第二段是优化的sql,想问下为什么第一段会那么慢?
INFO_TB_XXX_LIVE 数据量300万+,INFO_TB_XXX 数据量800万+
第一段
select b.id,
(select count(l.id) from INFO_TB_XXX_LIVE l where l.build_id = b.id and l.del_flag = '0' and l.LIVE_EDATE IS NULL) as LIVE_COUNT
from INFO_TB_XXX b
第二段
select b.id id,
count(l.id) LIVE_COUNT
from INFO_TB_XXX b
join INFO_TB_XXXX_LIVE l on l.build_id = b.id and l.del_flag = '0' and l.LIVE_EDATE IS NULL
group by b.id
两段sql结果是一样的,
但是还不清楚第一段为啥那么慢?