horike

We adopted the Elasticsearch to search engine. You can also search, such as post title, post content, custom fields by replacing the WordPress of standard search function. The accuracy of the search feature dramatically improves in WordPress.
<h4>Setting</h4>
<ul>
<li>In the <a href="https://wordpress.org/plugins/wp-simple-elasticsearch/screenshots/" rel="ugc">screenshoot section</a> you can look on the first screenshot. </li>
<li>Require setting to <code>Elasticsearch Endpoint</code>,<code>Port</code>,<code>index</code>,<code>type</code> and push <code>Save Changes</code>. It is default search target <code>post title</code>, <code>post content</code>.</li>
<li>Please push <code>Post Data sync to Elasticsearch</code>. So Posts data are sent to Elasticsearch.</li>
</ul>

WP Simple Elasticsearch

This plugin is that WordPress of standard search using ?s=xxx to replace WordPress DB with Elasticsearch.

Okamoto Hidetaka

This plugin gives you Related Item Widget and function.
If you want to show Related Item List, you can do following actions:
<ul>
<li>Add <code>Elasticommerce Related Widgets</code> in your widgets area.</li>
<li>Add <code>escr_related_item();</code> function in your theme or plugins.</li>
<li>If you only want to get the data, you can use <code>escr_get_related_item();</code> function.</li>
</ul>
Notice 
This plugin requires WooCommerce Plugin , so please install it. If WooCommerce Plugin is not installed or activated, this plugin will not work.
Required 
This Plugin cannot use Elasticsearch version 2.x. Please use Elasticsearch version 1.5 instead.

Elasticommerce Related Items

You can get good Related Item List powered by Elasticsearch.

WP Simple Elasticsearch vs Elasticommerce Related Items