docs: document live view TTL, retention and Parquet conversion

[puzpuzpuz]

Summary

Documents the durable-tier DDL that questdb/questdb#7612 adds to live views.

New page: ALTER LIVE VIEW. The statement had no reference page at all, so it covers all five clauses: SET TTL, DROP PARTITION, CONVERT PARTITION between native and Parquet, and the pre-existing RESUME WAL and SUSPEND WAL. Beyond the grammar it states what a live view being a derived object costs an operator:

• the change is applied asynchronously by the refresh worker, so FLUSH EVERY is a cadence and not a deadline;
• rows removed from disk can still be served from the in-memory tier until it is rebuilt, so neither DROP PARTITION nor SET TTL makes data unreadable at a known point in time;
• a manual DROP PARTITION is undone by any recovery that recomputes output over the dropped period, which is why SET TTL is the retention that survives recovery;
• an out-of-order base commit decodes and re-encodes every Parquet partition its correction range covers, so a view over a base table with frequent late writes is a poor Parquet candidate;
• WITH (bloom_filter_columns = ...) is not stored, and a live view has no per-column PARQUET() metadata to fall back on, so bloom filters last only until the partition is next rewritten.

It also lists the clauses a live view rejects (including FORCE DROP PARTITION), the permission each clause is authorized with, and how a replica applies a relayed statement to its own copy of the view.

CREATE LIVE VIEW: the TTL clause, its partition-size granularity rule, and what a retention window does during the initial seed.

Concept pages: a retention and storage format section on live views, replacing the now-false "no TTL on the view" limitation, plus the replication behaviour of these statements; and a live views subsection on the TTL page next to materialized views.

Unrelated correction: the in-place Parquet conversion example named the option bloom_filter_fpp and left the value unquoted, so it fails as written. ALTER TABLE ... CONVERT PARTITION parses fpp and needs a quoted value; bloom_filter_fpp is the COPY ... TO spelling.

Open question

The SET TTL section documents Enterprise as it behaves today: EntSqlCompilerImpl.alterTableOrMatViewSetTtl lets only a materialized view or a system table through, so a non-zero ALTER LIVE VIEW ... SET TTL is rejected with TTL is not supported on Enterprise tables; use a storage policy instead, while the CREATE LIVE VIEW ... TTL clause is accepted. Whether live views follow the materialized-view rule (TTL allowed) or the table rule (storage policy) is still open on the feature branch. That caution block needs a revisit before this merges.

Dependencies

• feat(core): accept TTL, DROP PARTITION and CONVERT PARTITION on live views questdb#7612, which adds the SQL. Do not merge before that ships.

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