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You run two databases because each one is locked to a single storage format.

Elasticsearch gives you inverted indexes — great for full-text search, terrible for aggregations at scale. ClickHouse gives you columnar storage — great for analytics, but its “full-text search” is a bloom filter that can’t rank results.

So you run both. You ingest the same data twice, store it twice, pay for it twice. You manage two clusters, two on-call rotations, two sets of expertise. And every time your workload changes, you manually re-tune both.