Problems with parameterizing timestamp references in PostgreSQL in Perl - sql

Question

I am trying to use a parameterized query in a Perl script to get some timestamps back from a Postgres database. Here's a cut-and-dried example, solely for pedagogical purposes.
I've defined $start_date and $end_date as timestamps and intervals:
my $start_date = "current_timestamp - interval '6 hours'";
my $end_date = "current_timestamp";
I use the following to submit to the database, with $dbh defined earlier:
my $sql = "SELECT cast(? as timestamp), cast(? as timestamp)";
my $sth = $dbh->prepare($sql);
$sth->execute($start_date, $end_date);
When I do this, I get a somewhat confusing error.
DBD::Pg::st execute failed: ERROR: date/time value "current" is no longer supported
I understand that current hasn't been supported in PG since 7.2, but I'm not using that. I'm using current_timestamp, which is supported, AFACT. To wit, if I enter into psql:
select (cast(current_timestamp - interval '6 hours' as timestamp), cast(current_timestamp as timestamp));
the result is what I expect (two timestamps, the former six hours previous to the latter).
I could also use now() rather than current_timestamp. I can use it in the following way:
my $start_date = "now() - interval '6 hours'";
my $end_date = "now()";
When I try to run the query in perl, I get the following error:
DBD::Pg::st execute failed: ERROR: invalid input syntax for type timestamp: "now() - interval '6 hours'"
Yet, the query:
select (cast(now() - interval '6 hours' as timestamp), cast(now() as timestamp));
gives me the expected result.
I am quite flummoxed.

Answer 1

The problem is that a SQL placeholder doesn't represent an expression, but a single value. And that value can't be a function. You could do something like:
my $start_date = "6 hours";
my $sql = "SELECT current_timestamp - cast(? as interval), current_timestamp";
my $sth = $dbh->prepare($sql);
$sth->execute($start_date);
What you're doing in Perl is equivalent to doing this in psql:
select (cast('current_timestamp - interval ''6 hours''' as timestamp), cast('current_timestamp' as timestamp));

Answer 2

To make the windows of your queries a bit more flexible:
$sth = $dbh->prepare(<<__eosql);
SELECT * FROM tbl
WHERE ts BETWEEN current_timestamp - ? * CAST('1 ' || ? AS INTERVAL)
AND
current_timestamp;
__eosql
$sth->execute(6, 'hour');
$sth->execute(10, 'day');
$sth->execute(1, 'week');
# etc.
When you introduce fixed time points, you could do something too clever like ... WHERE COALESCE(?, current_timestamp) ... and remember that an undef parameter defaults to the current time. However, I'd probably write and prepare a separate query.