Re: [SQL] md5 checksum of a previous row

You just need :
- to define the way to find the previous row
- an BEFORE INSERT trigger to compute the checksum, read up some pg/plsql or SQL to write your function. To read the row as a whole just use the table name without column in the select
- two rules (update / delete) to do nothing

On 13/11/2017 08:15, Iaam Onkara wrote:

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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Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt