What changed
PostHog announced on October 27, 2026 that a storage migration taking effect November 30 will change retention and query behaviour. Raw person-update instructions ($set, $set_once, $unset and $group_set) and selected SDK, replay and feature-flag diagnostic properties will be retained for 60 days from event receipt, rather than indefinitely. Feature flag evaluation events ($feature_flag_called) move to a dedicated flag_evaluations table with 90-day retention and no longer appear in ordinary all-events batch exports. Queries using SELECT *, SAMPLE or FINAL against these flag calls will error. PostHog reports approximately 35% faster average queries and around twice the speed for common session-based queries in its own replay tests.
Why it matters
Teams relying on historical event properties, SQL, exports, anomaly investigations or feature-flag audit trails could silently lose access to some diagnostic details or experience query errors. The migration is not a general purge of all events: normal event properties and resolved person properties remain under their existing retention rules. Builders need to identify the affected special fields and workflows before November 30.
The 60-day rule affects raw instructions, not resolved person data
Raw $set, $set_once, $unset and $group_set instructions on events expire 60 days after receipt, alongside certain $sdk_debug_*, $feature_flag_request_id, replay diagnostic and SDK transport properties. The resulting person properties remain on person records and ordinary events. Historical records received before roughly October 1 lose these raw instructions when the November 30 migration takes effect. The 60-day clock uses receipt time, not the event timestamp.
Flag calls move out of the events table
PostHog will keep $feature_flag_called in a dedicated flag_evaluations table for 90 days. Standard insights, funnels, cohorts and supported HogQL queries continue to access flag calls, subject to the shorter window. However, queries that use SELECT *, SAMPLE or FINAL for flag calls will error, and exports of all events will stop including new flag calls. Some Paths reports also stop counting them. Workflows and destinations explicitly triggered by flag calls continue to function.
Some properties disappear, others move
Deprecated flag payload, active-flags, product-tour and survey SDK state fields stop being stored as before. Raw LLM input/output payloads were already moved into an AI observability table with a 30-day retention window; PostHog says removing legacy copies from events does not change the current AI observability product. The company distinguishes these special fields from ordinary custom event properties.
Performance claims are internal benchmarks
PostHog says its new typed, non-duplicated storage is about 35% faster on average, roughly 50% faster for event-property filtering, and around 2x faster for session queries. These results come from PostHog replaying its own production queries, not an independent benchmark. Queries spanning many custom properties may improve less or become slightly slower.