Re: Speed of exist

Hi Andy,

I've tried that with the same result. One subquery works beautifully,
two subqueries with an OR and it starts to do a sequential scan...

Thanks,

Bastiaan Olij

On 19/02/13 6:31 PM, Andy wrote:
> Limit the sub-queries to 1, i.e. :
>
> select 1 from Table2 where Table2.ForeignKey = Table1.PrimaryKey fetch
> first 1 rows only
>
> Andy.
>
> On 19.02.2013 07:34, Bastiaan Olij wrote:
>> Hi All,
>>
>> Hope someone can help me a little bit here:
>>
>> I've got a query like the following:
>> --
>> select Column1, Column2, Column3
>> from Table1
>> where exists (select 1 from Table2 where Table2.ForeignKey =
>> Table1.PrimaryKey)
>> or exists (select 1 from Table3 where Table3.ForeignKey =
>> Table1.PrimaryKey)
>> --
>>
>> Looking at the query plan it is doing a sequential scan on both Table2
>> and Table3.
>>
>> If I remove one of the subqueries and turn the query into:
>> --
>> select Column1, Column2, Column3
>> from Table1
>> where exists (select 1 from Table2 where Table2.ForeignKey =
>> Table1.PrimaryKey)
>> --
>>
>> It is nicely doing an index scan on the index that is on
>> Table2.ForeignKey.
>>
>> As Table2 and Table3 are rather large the first query takes minutes
>> while the second query takes 18ms.
>>
>> Is there a way to speed this up or an alternative way of selecting
>> records from Table1 which have related records in Table2 or Table3 which
>> is faster?
>>
>> Kindest Regards,
>>
>> Bastiaan Olij
>>
>>
>>
>