PostgreSQL: Create an index on timestamp::DATE
<p>I am creating a summary table that sums up all events in a given day.</p>

<pre><code>INSERT INTO graph_6(
  day,
  event_type,
  (SELECT COUNT(*) FROM event e 
                  WHERE event_type = e.event_type 
                  AND creation_time::DATE = sq.day)
FROM event_type
CROSS JOIN
    (SELECT generate_series(
                (SELECT '2014-01-01'::DATE),
                (SELECT '2014-01-02'::DATE),
                '1 day') as day) sq;
</code></pre>

<p>The <code>creation_time</code> column is indexed:</p>

<pre><code>CREATE INDEX event_creation_time_date_idx ON event USING BTREE(creation_time);
</code></pre>

<p>However, the query runs a pretty long time even when only querying two days of data with a handful of events (January 1-2 2014).</p>

<p>The <code>EXPLAIN</code> on the query is pretty grim - it runs a sequential scan on the <code>event</code> table, not utilizing the index at all:</p>

<pre><code>-&gt;  Seq Scan on event e_1  (cost=0.00..12557.39 rows=531 width=38)
Filter: ... AND ((creation_time)::date = (generate_series(($12)::timestamp with time zone, ($13)::timestamp with time zone, '1 day'::interval))))
</code></pre>

<p>I assume this is because we compare a casted value - <code>creation_time::DATE</code>, not <code>creation_time</code>. I have tried indexing the cast:</p>

<pre><code>CREATE INDEX event_creation_time_date_idx ON event USING BTREE(creation_time::DATE);
</code></pre>

<p>But got an error:</p>

<pre><code>ERROR: syntax error at or near "::"
</code></pre>

<p><strong>Is there a way to utilize PostgreSQL indices on a timezone column casted to DATE?</strong></p>