Using named placeholders with interval fails in PHP and PostgreSQL - postgresql

Question

Okay I've confirmed this works explicitly with PHP.
$ php --version
PHP 5.6.16 (cli) (built: Dec 30 2015 15:09:50) (DEBUG)
<pdo version>
pdo_pgsql
PDO Driver for PostgreSQL enabled
PostgreSQL(libpq) Version 9.4.0
Module version 1.0.2
Revision $Id: fe003f8ab9041c47e97784d215c2488c4bda724d $
I would like to recreate the following SQL in PHP using PDO:
UPDATE relationships SET status = 4 WHERE created > NOW() - interval '2 seconds';
This script is working:
<?php
$db = new PDO('pgsql:dbname=db;host=localhost;user=stevetauber');
$stmt = $db->prepare("UPDATE relationships SET status = 4 WHERE created > NOW() - interval '?'");
$stmt->execute(array("2 seconds"));
Here it is with named placeholders:
<?php
$db = new PDO('pgsql:dbname=db;host=localhost;user=stevetauber');
$stmt = $db->prepare("UPDATE relationships SET status = 4 WHERE created > NOW() - interval ':blah'");
$stmt->execute(array(":blah" => "2 seconds"));
Which gives this error:
Warning: PDOStatement::execute(): SQLSTATE[HY093]: Invalid parameter number: :blah in ... line 5
Now according to PHP documentation,
Example #6 Invalid use of placeholder:
<?php
$stmt = $dbh->prepare("SELECT * FROM REGISTRY where name LIKE '%?%'");
$stmt->execute(array($_GET['name']));
// placeholder must be used in the place of the whole value
$stmt = $dbh->prepare("SELECT * FROM REGISTRY where name LIKE ?");
$stmt->execute(array("%$_GET[name]%"));
?>
Here is the updated code:
<?php
$db = new PDO('pgsql:dbname=db;host=localhost;user=stevetauber');
$stmt = $db->prepare("UPDATE relationships SET status = 4 WHERE created > NOW() - :blah");
$stmt->execute(array(":blah" => "interval '2 seconds'"));
Which yields these DB errors (no script errors):
ERROR: operator does not exist: timestamp with time zone > interval at character 51
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.
STATEMENT: UPDATE relationships SET status = 4 WHERE created > NOW() - $1
PDO is doing something weird here though because:
# select NOW() - interval '2 seconds' as a , pg_typeof(NOW() - interval '2 seconds') as b;
a | b
-------------------------------+--------------------------
2015-12-30 18:02:20.956453+00 | timestamp with time zone
(1 row)
So how do I use named placeholders with PostgreSQL and interval?

Answer 1

Placeholders are for pure values, not for values decorated with units (or with anything else).
To express interval '2 seconds' in a placeholder, there are two options:
in the query, write :secs * interval '1 second'
and bind :secs to a number in php
or write: cast(:mystring as interval), and bind :mystring to the string '2 seconds'. It will be interpreted dynamically through the explicit cast.
When experimenting with the psql command line client to compare with the PDO driver, use the PREPARE and EXECUTE SQL statements with postgres native $N placeholders, as opposed to having the parameters values already written literally in the query. This will match what the PHP driver is essentially doing when PDO::ATTR_EMULATE_PREPARES is set to false.
In the last part of you question, when trying this in psql (your query, just simplified to not need a table):
select now() > now() - interval '2 seconds';
it does work and returns 't' (true).
But if you tried that:
prepare p as select now() > now() - $1;
if would fail with
ERROR: operator does not exist: timestamp with time zone > interval
which is the same error as with PDO's prepare/execute.
On the other hand, this does work:
=> prepare p as select now() > now() - interval '1 second'*$1;
PREPARE
=> execute p(2);
?column?
----------
t