Re: md5 checksum of a previous row

you will have to still specify m2.id and m2.created_at but having to hard
code the column names is not ideal as any schema change will require a
change in the query. Hence my comment earlier "Seems to me this should be
a first class function in PostgreSQL, but its not."

Thanks,
Onkara

On Mon, Nov 13, 2017 at 1:42 AM, MS (direkt) <
martin(dot)stoecker(at)stb-datenservice(dot)de> wrote:

> select m2.* from .... will do the job in my example
>
>
> Am 13.11.2017 um 08:39 schrieb Iaam Onkara:
>
> Thanks that is very helpful.
>
> Now is there a way to fetch previous record without having to specify the
> different column names? Seems to me this should be a first class function
> in PostgreSQL, but its not.
>
> On Mon, Nov 13, 2017 at 1:31 AM, MS (direkt) <martin(dot)stoecker(at)stb-
> datenservice.de> wrote:
>
>> Hi,
>>
>> you can easily join the preceeding row, e.g.
>>
>> select sub.id, sub.created_at, preceedingid, m2.* from (
>> select m.id, m.created_at, lag(m.id) over(order by m.created_at) as
>> preceedingid from test m
>> order by m.created_at) as sub
>> left join test m2 on m2.id=sub.preceedingid order by sub.created_at;
>>
>> Regards, Martin
>>
>>
>> Am 13.11.2017 um 07:15 schrieb Iaam Onkara:
>>
>> Hi,
>>
>> I have a requirement to create an tamper proof chain of records for audit
>> purposes. The pseudo code is as follows
>>
>> before_insert:
>> 1. compute checksum of previous row (or conditionally selected row)
>> 2. insert the computed checksum in the current row
>> 3. using on-update or on-delete trigger raise error to prevent
>> update/delete of any row.
>>
>> Here are the different options that I have tried using lag and md5
>> functions
>>
>> http://www.sqlfiddle.com/#!17/69843/2
>>
>> CREATE TABLE test
>> ("id" uuid DEFAULT uuid_generate_v4() NOT NULL,
>> "value" decimal(5,2) NOT NULL,
>> "delta" decimal(5,2),
>> "created_at" timestamp default current_timestamp,
>> "words" text,
>> CONSTRAINT pid PRIMARY KEY (id)
>> )
>> ;
>>
>> INSERT INTO test
>> (value, words)
>> VALUES
>> (51.0, 'A'),
>> (52.0, 'B'),
>> (54.0, 'C'),
>> (57.0, 'D')
>> ;
>>
>> select
>> created_at, value,
>> value - lag(value, 1, 0.0) over(order by created_at) as delta,
>> md5(lag(words,1,words) over(order by created_at)) as the_word,
>> md5(textin(record_out(test))) as Hash
>> FROM test
>> ORDER BY created_at;
>>
>> But how do I use lag function or something like lag to read the previous
>> record as whole.
>>
>> Thanks,
>> Onkara
>> PS: This was earlier posted in 'pgsql-in-general' mailing list, but I
>> think this is a more appropriate list, if I am wrong I am sorry
>>
>>
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