Clarify hypercore migration guidance for existing hypertables

[atovpeko]

Describe your changes

Adds an "Enable hypercore on an existing hypertable" section to setup-hypercore.mdx covering the two recommended paths for converting a populated hypertable to the columnstore:

• Hands-off: add_columnstore_policy (notes the policy runs single-threaded and may lag on a large backlog).
• Manual: convert_to_columnstore per chunk, with an explicit parallel-sessions sample for faster initial migration.

Also adds a warning callout explaining that conversion takes an ExclusiveLock on the chunk and contends with concurrent backfill — links to the existing pause-backfill-reconvert workflow on convert_to_rowstore.

Extends the convert_to_columnstore reference page with a bulk-migration sample showing the same parallel pattern.

Why: Review feedback flagged that the docs were missing recommended paths for migrating existing hypertables and didn't mention lock contention between conversion and backfill. Behavior was verified against the TimescaleDB 2.27.0 source (tsl/src/compression/api.c:658 for ExclusiveLock, sql/policy_internal.sql:93-121 for the serial policy loop) and end-to-end on a live Tiger Cloud service.

Affected pages

• /build/columnar-storage/setup-hypercore
• /reference/timescaledb/hypercore/convert_to_columnstore

Related Issues

Issue: #EDU-131

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Project	Deployment	Actions	Updated (UTC)
tiger-data-docs	Error		May 27, 2026 12:56pm