MySQL search engine or ElasticSearch engine, which one to use ?
Thanks again for insights.
I did some extensive test and yes on big Tiki, ElasticSearch is definitely faster and better.
Simple word search may not show you the benefit of using ElasticSearch over MySQL but more complex search and using related features will show the differences.
For example, indexing and searching for a term within a 72 page PDF stored in a Tiki file gallery will work fine and smoothly (fast, no need to rebuild the unified-search-index after uploading it) while, when using MySQL it is slow and somehow hazardous (some search attempt left me with a not found).