Re: 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 <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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Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt