How to get post IDs of all the posts in a given category

post-ids-by-category.php

@@ -0,0 +1,41 @@
+<?php
+/*
+Plugin Name: Get Post IDs by Category
+Plugin URI: http://wordpress.stackexchange.com/q/71471/6035
+Description: Get all post ids for a given category.
+Author: Christopher Davis
+Author URI: http://christopherdavis.me
+License: GPL2
+
+    Copyright 2012 Christopher Davis
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License, version 2, as 
+    published by the Free Software Foundation.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+!defined('ABSPATH') && exit;
+
+function wpse71471_get_post_ids($cat, $taxonomy='category')
+{
+    return get_posts(array(
+        'numberposts'   => -1, // get all posts.
+        'tax_query'     => array(
+            array(
+                'taxonomy'  => $taxonomy,
+                'field'     => 'id',
+                'terms'     => is_array($cat) ? $cat : array($cat),
+            ),
+        ),
+        'fields'        => 'ids', // only get post IDs.
+    ));
+}