coalesce seems to give strange results

Hi everyone,

Looks like I'm encountering some quirks with coalesce()...

> postgres=# select coalesce(null,0);
> coalesce
> ----------
> 0
> (1 row)
>
> postgres=# SELECT COALESCE(ROUND(EXTRACT(epoch FROM now()-query_start)),0) FROM pg_stat_activity WHERE current_query = '<IDLE> in transaction';
> coalesce
> ----------
> (0 rows)
>
> postgres=# select version();
> version
> -------------------------------------------------------------------------------------------------------------------
> PostgreSQL 8.4.2 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44), 64-bit
> (1 row)

Any ideas?
--Richard