Re: Identifying no-op length coercions

On Jun 21, 2011, at 9:58 PM, Noah Misch wrote:

>
> A pg_regress test needs stable output, so we would do it roughly like this:
>
>     CREATE TEMP TABLE relstorage AS SELECT 0::regclass AS oldnode;
>     ...
>     UPDATE relstorage SET oldnode =
>         (SELECT relfilenode FROM pg_class WHERE oid = 'test'::regclass);
>     ALTER TABLE test ALTER name TYPE varchar(65535);
>     SELECT oldnode <> relfilenode AS rewritten
>     FROM pg_class, relstorage WHERE oid = 'test'::regclass;
>
> I originally rejected that as too ugly to read.  Perhaps not.

Yes, your example is more appropriate. I think you can make it more
straightforward by getting rid of the temp table:

CREATE TABLE test(oldnode oid, name varchar(5));

INSERT INTO test(oldnode) SELECT relfilenode FROM pg_class WHERE
oid='test'::regclass;

ALTER TABLE test ALTER name TYPE varchar(10);

SELECT oldnode <> relfilenode AS rewritten FROM pg_class, test WHERE
oid='test'::regclass;



>
>> The only nitpick code-wise is these lines  in varchar_transform:
>>
>> +         int32        old_max = exprTypmod(source) - VARHDRSZ;
>> +         int32        new_max = new_typmod - VARHDRSZ;
>>
>> I have a hard time understanding why  VARHDRSZ is subtracted here, so I'd assume that's a bug.
>
> We track the varchar typmod internally as (max length) + VARHDRSZ.

Oh, right, haven't thought that this is a varchar specific thing.

Thank you,
Alexey.

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