Re: 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(at)matrix(dot)gatewaynet(dot)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(dot)stoecker(at)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(dot)stoecker(at)stb-datenservice(dot)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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>>
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>>
>>
>
> --
> Achilleas Mantzios
> IT DEV Lead
> IT DEPT
> Dynacom Tankers Mgmt
>
>