Target : What i'm trying to achieve is to restore a partial backup of a database (only certain rows that satisfied certain conditions have been backed up).
Problem : Since between the partial-backup and the restore may pass some time, and certain records could be deleted, I need to perform an Upsert(Update if record exist, Insert if record doesn't exists). This operation in Mysql would have be done with REPLACE, but in PostgreSQL it doesn't exist. After some research I've found a good substitute of it(correct me if i'm wrong) in this function :
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- Do nothing, and loop to try the UPDATE again.
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
The problem with this is that I need to make dynamic the input parameters of the function, and also the UPDATE and the INSERT statements, in order to adapt it for every record and every table i'm going to upsert.
Question :
- is it possible to integrate plpgsql functions in bash scripts in order to makes them more dynamic?
Best Answer
As you were hinted in comments above, your RBAR approach might be very inefficient. Consider the suggestions there.
Also, I am not going into details about the different approaches of
UPSERT
, as it is a very broad topic, especially when one wants to do concurrency-safe. PostgreSQL 9.5 helps a lot in this regard.So, to your actual question: it is not directly possible writing PL/pgSQL 'commands' interspersed with bash, but the task is definitely solvable.
For this, you need to define a function like this (left out the complexity to show my point more concisely):
Please note that this is fully dynamic in that you can choose any table, match against any column of it and modify any (other) column. The type of the key (
integer
) and the value (text
) are fixed, though.Now you can call this from a shell script like
If you are still RBARing, this will produce an additional overhead of connecting to the database for every single change, so be patient (or set up a connection pool).
Notes:
quote_ident()
, which is the way to go if you are concatenating the dynamic statement. In newer versions (from 9.1), one would use theformat()
function to avoid the mess: