It is the method or technique for the search of a single or collection of computer-stored documents in a full-text database. It can be easily differentiated from searches based on the metadata or on parts of the original texts represented in databases.
Although PostgreSQL is not as advanced as compared to the Elasticsearch and SOLR, both of these are specific for full-text search tools. Whereas in PostgreSQL, full-text search is only a feature and it is a pretty good one.
