Conditionally construct schema depending on database version in PostgreSQL - postgresql

Question

Assume the following table and custom range type:
create table booking (
identifier integer not null primary key,
room uuid not null,
start_time time without time zone not null,
end_time time without time zone not null
);
create type timerange as range (subtype = time);
In PostgreSQL v10, you can do:
alter table booking add constraint overlapping_times
exclude using gist
(
room with =,
timerange(start_time, end_time) with &&
);
In PostgreSQL v9.5/v9.6, you have to manually cast the uuid column as gist_btree does not support uuid:
alter table booking add constraint overlapping_times
exclude using gist
(
(room::text) with =,
timerange(start_time, end_time) with &&
);
I would like to support v9.5, v9.6 and v10 for my customers. Is there a way to conditionally add the above constraint in the same .sql file, depending on the version of the current database?

Answer 1

You can use dynamic sql
For example:
do
$block$
declare
l_version text;
begin
select setting into l_version from pg_settings where name = 'server_version';
execute
format(
$script$
alter table booking add constraint overlapping_times
exclude using gist
(
%s with =,
timerange(start_time, end_time) with &&
)
$script$,
case when (l_version like '9.5.%' or l_version like '9.6.%') then '(room::text)' else 'room' end
)
;
end;
$block$
language plpgsql;

Answer 2

Here is a proof of concept:
#!/bin/sh
#THE_HOST="192.168.0.104"
THE_HOST="192.168.0.101"
get_version ()
{
psql -t -h ${THE_HOST} -U postgres postgres <<OMG | awk -e '{ print $2; }'
select version();
OMG
}
# ############################################################################
# main
#
# - Connect to database to retrieve version
# - use the retrieved version to create a symlink "versioned" to
# one of our subdirs
# - call an sql script that includes this symlink/some.sql
# symlinking to a non-existing directory will cause a dead link
# (, and the script to fail.)
# ergo: there should be a subdir "verX.Y.Z" for every supported version X.Y.Z
# ############################################################################
pg_version=`get_version`
#echo "version=${pg_version}"
rm versioned
ln -fs "ver${pg_version}" versioned
if [ -f versioned/alter.sql ]; then
echo created link versioned to "ver${pg_version}"
else
echo "version ${pg_version} not supported today..."
echo "Failed!"
exit 1
fi
psql -t -h ${THE_HOST} -U postgres postgres <<LETS_GO
\i common/create.sql
\i versioned/alter.sql
\echo done!
LETS_GO
#eof