Re: Optimizing query?

2013/1/31 Vincent Veyron <vv(dot)lists(at)wanadoo(dot)fr>:
> Le jeudi 31 janvier 2013 à 09:49 +0100, Pavel Stehule a écrit :
>
>> any join where result is related to some function result can be very
>> slow, because estimation will be out and any repeated function
>> evaluation is just expensive.
>>
>
> Hi Pavel,
>
> Thank you for the correction. Since we're at it, I have a question
> regarding functions in a query :
>
> Suppose I have a query of the form
>
> SELECT my_function(column_1), column_2
> FROM my_table
> GROUP BY my_function(column_1)
> ORDER BY my_function(column_1);
>
> where my_function is a user defined function.
>
> How many times is the function computed?

if function is stable or immutable, then once per row

Pavel

>
>
>
>
> --
> Salutations, Vincent Veyron
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