CODEWITHSUNDEEP
postgresqlpsql
user272735
user272735

Reputation: 10648

How to bind a SQL query return value to a psql variable?

Background: I'm writing my first pgTAP test cases for a PL/pgSQL function and starting small with psql test scripts. No problems on that but I run into a small annoyance on psql variables.

In my test scripts I first dump quite a bit of test data to a relevant tables and later refer to the data with a primary keys that were generated by sequences. I'd find it handy to be able to create a variable that would contain the primary key. This is what I'm looking for:

scalasb=> \set the_id (select currval('id_data_id_seq'))
scalasb=> \echo :the_id
54754
scalasb=>

But this is what I get:

scalasb=> \set the_id (select currval('id_data_id_seq'))
scalasb=> \echo :the_id
(selectcurrval('id_data_id_seq'))
scalasb=>

I have a workaround (please see the example below), but it looks like psql variables are not the right tool for this job. Or they are just a different of what I've used with Oracle sqlplus bind variables ...

So my question: how I can bind the return value of a SQL query into a variable in psql script ?

I'm working on a linux with 9.1.

-- this is a simplified example to illustrate the problem
begin;

create table id_data(id serial primary key, data text not null);

create or replace function get_text(p_id bigint)
returns text
as $$
declare
  v_data constant text := data from id_data where id = p_id;
begin
  return v_data;
end;
$$ language plpgsql;

insert into id_data(data) values('lorem ipsum');

-- this works correctly but is a rather verbose (especially when one have
-- more of these, in the same query/function)
select get_text((select currval('id_data_id_seq')));

-- instead I'd like to set the id to a variable and use that but this
-- seems to be impossible with psql, right ?
\set the_id (select currval('id_data_id_seq'))
\echo First try: :the_id
--select get_text(:the_id); -- this will fail

-- this works and reveals psql variables' true nature - they are just a
-- textual replacements
\set the_id '(select currval(\'id_data_id_seq\'))'
\echo Second try: :the_id
select get_text(:the_id);

rollback;

Upvotes: 4

Views: 3153

Answers (1)

Pavel Stehule
Pavel Stehule

Reputation: 45900

I afraid so you cannot do it - I wrote some patches for psql, but it is not in core

you can use workaround:

postgres=# \set myvar `psql -A -t -c "select version()" postgres `
postgres=# \echo :myvar
PostgreSQL 9.1.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5), 64-bit

Note: PostgreSQL 9.3 (2013) and newer supports \gset command. With this command, the mentioned workaround is baseless:

(2023-04-02 17:03:00) postgres=# select version() as myvar \gset
(2023-04-02 17:03:06) postgres=# \echo :myvar
PostgreSQL 16devel on x86_64-pc-linux-gnu, compiled by gcc (GCC) 13.0.1 20230318 (Red Hat 13.0.1-0), 64-bit

Upvotes: 6

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