對於exists和in,當exists裡面巢狀in的時候,如果in語句裡面只有乙個元素的情況下,這時候索引是有效的.如果多於乙個元素,這種情況下索引會失效.下面寫乙個簡單的例子來驗證以上說法.
sql1:
select 1 from pm_measurement_recalculation reca
where
exists (
select
operating_date
from
pm_measurement_reading reading
where
reading.measurement_id in(
-- 10000828, 10000670, 10000671,215001368
10000828,10000670,10000671) )
執行計畫圖:(索引失效)
執行計畫圖:
準備優化的原sql:
update
pm_measurement_recalculation reca
setreca.from_operating_date=( select
min(temptable.operating_date) as operating_date
from
( select
min(r.operating_date) as operating_date
from
pm_measurement_reading r
where
r.measurement_id in (
10000828, 10000670, 10000671, 215001368
) group by
r.operating_date ) as temptable )
where
reca.measurement_id in (
10000828, 10000670, 10000671,215001368
) and
exists (
select operating_date from pm_measurement_reading reading
where reading.measurement_id in(
10000828, 10000670, 10000671,215001368 ) )
執行計畫圖
優化後的sql:
update
pm_measurement_recalculation reca
setreca.from_operating_date=( select
min(temptable.operating_date) as operating_date
from
( select
min(r.operating_date) as operating_date
from
pm_measurement_reading r
where
r.measurement_id in (
10000828, 10000670, 10000671, 215001368
) group by
r.operating_date ) as temptable )
where
reca.measurement_id in (
10000828, 10000670, 10000671,215001368
) and
exists (
select operating_date from pm_measurement_reading reading
where reading.measurement_id in(
select
reading.measurement_id
from
pm_measurement_reading reading
where
reading.measurement_id in(
10000828, 10000670, 10000671,215001368
)group by reading.measurement_id
fetch first 1 rows only
) )
執行計畫圖:
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