Re: Gin indexes on intarray is fast when value in array does not exists, and slow, when value exists

On Tue, Nov 8, 2016 at 8:43 PM, otar shavadze <oshavadze(at)gmail(dot)com> wrote:

> I have table with 500 000 rows, I have int[] column "my_array" in this
> table, this array column contains minimum 1 and maximum 5 different values.
>

you didn't show us postgres version.

>
> I have GIN index on my_array column:
>
> * "CREATE INDEX idx ON table_name USING GIN(my_array gin__int_ops)"*
>
> Then I use this query: "*SELECT * FROM table_name WHERE my_array @>
> '{3}'::integer[] ORDER BY id LIMIT 50"*
>
> Execution time of this query is approximately 500-1000 ms. Then if I drop
> gin index "*idx*", query works extremely fast, less than 20 ms.
>

explain analyze would help us to see the problem.

>
> But, if I search value, which does not exists at all, for example no one
> array not contains number "77" and I search: * "WHERE my_array @>
> '{77}'::integer[]" *, then using gin index is much better and fast, (less
> than 20 ms), but without index, query takes 500-1000 ms.
>
>
> So, what to do? For values which does not in any one rows, using index is
> much better, but for values, which are at least in several rows, using
> index, slows down performance.
>
> Can somehow make, that searching was always fast (when value exist in
> array and when not)
>
>
>