How do I store date Variable in Postgres function? - sql

Question

So I am working to create a function that will delete the 1 month worth records from a table. The table is in postgres. As postgres does not have stored procedures I am trying to declare a function with the logic that will insert the 1 month records into a history table and then delete the records from the live table. I have the following code :
CREATE FUNCTION DeleteAndInsertTransaction(Integer)
RETURNS Void
AS $Body$
SELECT now() into saveTime;
SELECT * INTO public.hist_table
FROM (select * from public.live_table
WHERE update < ((SELECT * FROM saveTime) - ($1::text || ' months')::interval)) as sub;
delete from public.live_table
where update < ((SELECT * FROM saveTime) - ($1::text || ' months')::interval);
DROP TABLE saveTime;
$Body$
Language 'sql';
So the above code compiles fine but when I try to run it by invoking it :- DeleteAndInsertTransaction(27) it gives me an
Error: relation "savetime" does not exist and I have no clue what is going on here.
If I take out the SELECT now() into saveTime; out of the function bloc and declare it before invoking the function then it runs fine but I need to store the current date into a variable and use that as a constant for the insert and delete and this is going against a huge table and there could be significant time difference between the insert and deletes. Any pointers as to what is going on here ?

Answer 1

select .. into .. is the deprecated syntax for create table ... as select ... which creates a new table.
So, SELECT now() into saveTime; actually creates a new table (named savetime), and is equivalent to: create table savetime as select now(); - it's not storing something in a variable.
To store a value in a variable, you need to first declare the variable, then you can assign the value. But you can only do that in PL/pgSQL, not SQL
CREATE FUNCTION DeleteAndInsertTransaction(p_num_months integer)
returns void
as
$Body$
declare
l_now timestamp;
begin
l_now := now();
...
end;
$body$
language plpgsql;
To insert into an existing table you need
insert into public.hist_table
select *
from public.live_table.
To select the rows from the last x month, there is no need to store the current date and time in a variable to begin with. It's also easier to use make_interval() to generate an interval based on a specified unit.
You can simply use
select *
from live_table
where updated_at <= current_date - make_interval(mons => p_pum_months);
And as you don't need a variable, you can actually do all that with a language sql function.
So the function would look something like this:
CREATE FUNCTION DeleteAndInsertTransaction(p_num_months integer)
RETURNS Void
AS
$Body$
insert into public.hist_table
select *
from live_table
where updated_at < current_date - make_interval(months => p_pum_months);
delete from public.live_table
where updated_at < current_date - make_interval(months => p_pum_months);
$Body$
Language sql;
Note that the language name is an identifier and should not be quoted.
You can actually do the DELETE and INSERT in a single statement:
with deleted as (
delete from public.live_table
where updated_at <= current_date - make_interval(months => p_pum_months)
returning *
)
insert into hist_table
select *
from deleted;