2011/9/25 pasman pasmański <pasman.p@xxxxxxxxx>:
> I found second use case. Look at expression:
>
> where left(str,n)='value'
>
> function left(str,n) increase monotonically for str and n. With this
> feature it can use index on str.
>
> Classic index needs recreating.
>
these use cases are just theory - for example - this case should be
solved with immutable functions
I can use a functional index left(str, const) and use a query
where left(str, const) = left('value', const) and left(str, n) = 'value'
There are a theoretical cases, but these cases should be solved via
special data type and GiST index
Pavel
> 2011/9/25, Pavel Stehule <pavel.stehule@xxxxxxxxx>:
>> Hello
>>
>> what is a real use case?
>>
>> Regards
>>
>> Pavel
>>
>> 2011/9/25 pasman pasmański <pasman.p@xxxxxxxxx>:
>>> My english is not perfect, by accumulative i think about monotonically
>>> increasing function.
>>>
>>> It works that for clause WHERE f(x)=const:
>>> 1. Read root page of index_on_x and get x1 ... Xn
>>> 2. Calculate f(x1) ... f(xn) for this page
>>> 3. When f(x1)<=const<= f(xn) then x1 <= searched x <= xn and we can
>>> test smaller range (xlower, xgreater).
>>> 4. Otherwise no rows satisfy condition.
>>>
>>> Step 3 we repeat for current index's page and subpages until xlower =
>>> searched x = xgreater
>>>
>>>
>>> 2011/9/25, Tom Lane <tgl@xxxxxxxxxxxxx>:
>>>> =?ISO-8859-2?Q?pasman_pasma=F1ski?= <pasman.p@xxxxxxxxx> writes:
>>>>> I propose to add "accumulative" flag to a function definition. This
>>>>> flag would be set for function f(x) which is accumulative and
>>>>> immutable.
>>>>
>>>> Maybe you'd better define what you mean by "accumulative" ...
>>>>
>>>>> This flag allows to use an index on x for clauses containing f(x):
>>>>> where f(x) = const
>>>>> where f(x) > const
>>>>
>>>> ... because it's sure not clear how you would get that to work.
>>>>
>>>> regards, tom lane
>>>>
>>>
>>>
>>> --
>>> ------------
>>> pasman
>>>
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>>
>
>
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> ------------
> pasman
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