It's far from trivial I think. Let's have a look at the topic title of this discussion:
Is there any Extension for similar Question Like this..
If you perform a search using this phrase you get... a
Something went wrong answer (after a timeout I guess). That's more a deficit of the current search function, so implementing this as an extension to find similar topics would require a better search. But the search would struggle here even if it were more performant, as there are too many words in the search phrase, the majority of them with little to none meaning regarding a search.
You could analyse the words of the search phrase and weighten them invers proportionally to their frequency of occurrence. In this case, the terms "extension", "similar" and "question" would stand out. One could in addition search for "extension" and "similar qestion", i.e. grouping adjacent significant words, to get even better results.
But as I said, these are suggestions for the search function.
My experience with search: It's an impossible challenge to do this with the full text search built into the forum's mysql database if there are more than a few tenthousand posts. In the end you need dedicated software like Lucene / Solr / Elastic Search for a performant search.