Re: md5 checksum of a previous row

On 13/11/2017 09:47, Iaam Onkara wrote:
> you will have to still specify m2.id <http://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 <mailto: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 <mailto:martin(dot)stoecker(at)stb-datenservice(dot)de>> wrote:
>>
>> Hi,
>>
>> you can easily join the preceeding row, e.g.
>>
>> select sub.id <http://sub.id>, sub.created_at, preceedingid, m2.* from (
>> select m.id <http://m.id>, m.created_at, lag(m.id <http://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 <http://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 <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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