jojaba

This plugin is meant to add the word autocomplete in search input field by using the index stored in Relevanssi database. So you have to <a href="https://wordpress.org/plugins/relevanssi/" rel="ugc">install first Relevanssi</a> (free or premium version) to have it to work.
The JS script used to accomplish the autocompletion work has been developed by <a href="http://lea.verou.me/" rel="nofollow ugc">Lea Verou</a> and is called <a href="https://leaverou.github.io/awesomplete" title="Go to the Awesomplete homepage" rel="nofollow ugc">Awesomplete</a>. It is very simple to set up, lightweight and accessible.
<h4>List of functionalities</h4>
<ul>
<li>Settings available: styling, min characters typed to show the autocompletion box, max suggestions in autocompletion box.</li>
<li>Get the Relevanssi Indexed words to have the autcompletion list</li>
<li>Fast and lightweight</li>
<li>English and french languages included</li>
</ul>

Autocomplete For Relevanssi

Relevanssi author here. Please do not use this plugin in its current form. It's dangerous for performance and may cause server crashes. It will also slow down page loads in a rather terrifying manner.

Dangerous, do not use

Autocompletion functionality for WordPress search input when Relevanssi plugin is installed.

Mathieu Chartier

En français
WP-Advanced-Search fonctionne en anglais et en français 
WP-Advanced-Search fonctionne avec un code court <?php WP_Advanced_Search(); ?> ou le shortcode [advanced-search] à ajouter dans la page de recherche du thème WordPress (search.php). Tout est entièrement paramétrable dans les réglages de WordPress (sous-menu ‘Advanced Search’). Disponible aussi pour WordPress MU (multisites).

Quelques exemples d’options existantes :
<ul>
<li>Choix des colonnes de recherche</li>
<li>Ordre des résultats modulable</li>
<li>Utilisation d’un algorithme de pertinence pour le classement final</li>
<li>Plusieurs types de pagination (classique, par clic avec “trigger” ou scroll infini)</li>
<li>Mise en surbrillance ou non des mots recherchés</li>
<li>Correction automatique des recherches</li>
<li>Autocomplétion et ajout automatique de mots dans l’index</li>
<li>Gestion des blocs à afficher dans les résultats</li>
<li>Plusieurs thèmes disponibles</li>
<li>Possibilité de rechercher une expression précise (mots entre guillemets)</li>
<li>Possibilité de rechercher des articles placés dans des catégories précises</li>
<li>…</li>
</ul>
Failles et vulnérabilités analysées par Florian Hauser de <a href="https://www.code-white.com/en/" rel="nofollow ugc">www.code-white.com</a>, merci à lui !
For English people
WP-Advanced-Search works in English and in French 
WP-Advanced-Search works with a simple code <?php WP_Advanced_Search (); ?> to add to the search page of WordPress (search.php). Everything is fully configurable in the settings of WordPress (sub-menu ‘Advanced Search’). Now available for WordPress MU !

Some examples of existing options:
<ul>
<li>Choice search columns</li>
<li>Flexible order results</li>
<li>Using a relevance algorithm for the final classification</li>
<li>Several system to display results (pagination, trigger or infinite scroll)</li>
<li>Highlight search terms, or not</li>
<li>Automatic correction of searches</li>
<li>Autocomplete option and automatic addition of words in the index</li>
<li>Manage blocks to display in the SERP</li>
<li>Several templates for result pages</li>
<li>Ability to write an exact search with words in quotation marks</li>
<li>Opportunity to show the results of specific categories</li>
<li>…</li>
</ul>
Vulnerabilities analyzed by Florian Hauser from <a href="https://www.code-white.com/en/" rel="nofollow ugc">www.code-white.com</a>, thanks to him!

WordPress WP-Advanced-Search

Moteur de recherche complet et puissant entièrement modulable (complete and powerful search engine, fully scalable).

Autocomplete For Relevanssi vs WordPress WP-Advanced-Search