Re: md5 checksum of a previous row

@Achilleas I don't think I can avoid using lag. As inside the before
insert trigger I will need to read the previous row. Or am I
misunderstanding you?

On Nov 13, 2017 2:14 AM, "Achilleas Mantzios" <achill(at)matrix(dot)gatewaynet(dot)com>
wrote:

> On 13/11/2017 10:01, Iaam Onkara wrote:
>
> 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
>
>
> And since you are disabling deletes, updates, your best bet is to try and
> write the last committed row's md5 inside the trigger we were talking about
> in the very first post, in which you won't even need lag() .
>
>
> 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(dot)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
>>
>>
>
> --
> Achilleas Mantzios
> IT DEV Lead
> IT DEPT
> Dynacom Tankers Mgmt
>
>