Re: [SQL] md5 checksum of a previous row

Yes obviously I will need an extra column to store the checksum of the previous row. The difficultly I was having was not knowing how to read the whole of the previous row, which I just learned from Thomas and here is the updated fiddle http://www.sqlfiddle.com/#!17/69843/20

On Mon, Nov 13, 2017 at 1:57 AM, Achilleas Mantzios <achill@matrix.gatewaynet.com> wrote:

On 13/11/2017 09:47, Iaam Onkara wrote:

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."

lag() does not work with record type, only anyelement.
What keeps you from writing a trigger and doing smth like :
select md5(test::text)  from test ORDER BY created_at DESC LIMIT 1;
This will do the md5 on the whole row.
You should have an extra col to store that.

Thanks,

Onkara

On Mon, Nov 13, 2017 at 1:42 AM, MS (direkt) <martin.stoecker@stb-datenservice.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.stoecker@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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