[英]postgres: when index is used
just to better understand in which cases index is correctly used i would like to enumerate possible cases. 为了更好地了解在哪种情况下正确使用了索引,我想列举一些可能的情况。
let's assume we have a table with "a", "b", "c", "d" columns. 假设我们有一个包含“ a”,“ b”,“ c”,“ d”列的表。
we create an index on (a, b, c, d): 我们在(a,b,c,d)上创建索引:
create index my_index on table my_table (a, b, c, d)
is it used when: 是否在以下情况下使用:
1) 1)
where a=% and b=% and c=% and d=%
2) 2)
where a=% and b=%
3) 3)
where a=%
4) 4)
where b=% and c=% and d=%
5) 5)
where c=% order by b
6) 6)
where a=% and b=% and c=% order by case when d is not null then d else c end
7) let's assume now we have more column for the 7 point but the index only on (a, b, c, d) 7)假设现在我们有7点的更多列,但索引仅在(a,b,c,d)上
where a=% and b=% and c=% and d=% and e=% and f=% and g=%
MySQL has pretty good documentation explaining multi-column indexes. MySQL有很好的文档说明了多列索引。 The rules are basically the same across databases (although there is some more advanced stuff).
规则在数据库之间基本相同(尽管有一些更高级的东西)。
Of course, there are other factors -- such as what is going on in the from
clause, what columns are being selected, statistics about the tables, and so on. 当然,还有其他因素-例如
from
子句中发生的情况,选择了哪些列,有关表的统计信息等等。
The following is basic guidance: 以下是基本指导:
1) where a=% and b=% and c=% and d=%
1)
where a=% and b=% and c=% and d=%
Yes, as long as all the conditions are equality. 是的,只要所有条件都是平等的。 I don't know what happens with collation conflicts.
我不知道排序规则冲突会怎样。
2) where a=% and b=% 2)其中a =%,b =%
Yes, as long as all the conditions are equality. 是的,只要所有条件都是平等的。 I don't know what happens with collation conflicts.
我不知道排序规则冲突会怎样。
3) where a=% 3)其中a =%
Yes, as long as all the conditions are equality. 是的,只要所有条件都是平等的。 I don't know what happens with collation conflicts.
我不知道排序规则冲突会怎样。
4) where b=% and c=% and d=% 4)其中b =%,c =%和d =%
Probably not. 可能不是。 If used, the index would need to be scanned.
如果使用索引,则需要对其进行扫描。 This would be the case if the index covered the query.
如果索引覆盖了查询,就会是这种情况。
5) where c=% order by b 5)其中c = b的%阶
Probably not. 可能不是。
6) where a=% and b=% and c=% order by case when d is not null then d else c end 6)其中,当d不为null时,a =%且b =%且c =%依次排序,则d else c结束
Should be used for the where
clause. 应该用于
where
子句。 A sort will still be needed. 仍然需要某种排序。
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