Re: md5 checksum of a previous row

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
> <http://www.sqlfiddle.com/#%2117/69843/2>
>
> 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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