Re: Altering a column type - Most efficient way

Ow Mun Heng schrieb:
> On Thu, 2008-07-10 at 09:57 -0400, Alvaro Herrera wrote:
>
>> Ow Mun Heng wrote:
>>
>>
>>> This is what I see on the table
>>>
>>> NEW attypmod = -1
>>> OLD attypmod = 8
>>>
>> 8 means varchar(4) which is what you said you had (4+4)
>> -1 means unlimited size.
>>
>>
>
> This is cool.
>
> If it were this simple a change, I'm not certain why (I believe) PG is
> checking each and every row to see if it will fit into the new column
> definition/type.
>
> Thus, I'm still a bit hesitant to do the change, although it is
> definitely a very enticing thing to do. ( I presume also that this
> change will be instantaneous and does not need to check on each and
> every row of the table?)
>
> Thanks./
>
>

It should be safe, because the length limit is checked at insert/update
time, and internally, a varchar(20) is treated as something like this:
foo varchar(1000000000) check (length(foo) <= 20)

The change is done without re-checking all rows, and will not fail IF
the new size is longer than the old size.