I've seen Elasticsearch set up for applications that would have equal benefit from just using the postgresql db's full-text search they already have access to.

I've seen Elasticsearch set up for applications that would have equal benefit from just using the postgresql db's full-text search they already have access to. The additional complexity is usually incurred when the data in postgresql changes, and those changes need to be mirrored up to Elasticsearch. Elasticsearch obviously has its uses, but for some cases, postgresql's built in full-text search can make more sense.