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In commit727f68e0f5we added the ability to SELECT: * Individual elements of a map: `SELECT map_col[key]`. * Individual elements of a set: `SELECT set_col[key]` returns key if the key exists in the set, or null if it doesn't, allowing to check if the element exists in the set. * Individual pieces of a UDT: `SELECT udt_col.field`. But at the time, we didn't provide any way to retrieve the **meta-data** for this value, namely its timestamp and TTL. We did not support `SELECT TIMESTAMP(collection[key])`, or `SELECT TIMESTAMP(udt.field)`. Users requested to support such SELECTs in the past (see issue #15427), and Cassandra 5.0 added support for this feature - for both maps and sets and udts - so we also need this feature for compatibility. This feature was also requested recently by vector-search developers, who wanted to read Alternator columns - stored as map elements, not individual columns - with their WRITETIME information. The first four patches in this series adds the feature (in four smaller patches instead one big one), the fifth and sixth patches add tests (cqlpy and boost tests, respectively). The seventh patch adds documentation. All the new tests pass on Cassandra 5, failed on Scylla before the present fix, and pass with it. The fix was surprisingly difficult. Our existing implementation (from727f68e0f5building on earlier machinery) doesn't just "read" `map_col[key]` and allow us to return just its timestamp. Rather, the implementation reads the entire map, serializes it in some temporary format that does **not** include the timestamps and ttls, and then takes the subscript key, at which point we no longer have the timestamp or ttl of the element. So the fix had to cross all these layers of the implementation. While adding support for UDT fields in a pre-existing grammar nonterminal "subscriptExpr", we unintentionally added support for UDT fields also in LWT expressions (which used this nonterminal). LWT missing support for UDT fields was a long-time known compatibility issue (#13624) so we unintentionally fixed it :-) Actually, to completely fix it we needed another small change in the expression implementation, so the eighth patch in this series does this. Fixes #15427 Fixes #13624 Closes scylladb/scylladb#29134 * github.com:scylladb/scylladb: cql3: support UDT fields in LWT expressions cql3: document WRITETIME() and TTL() for elements of map, set or UDT test/boost: test WRITETIME() and TTL() on map collection elements test/cqlpy: test WRITETIME() and TTL() on element of map, set or UDT cql3: prepare and evaluate WRITETIME/TTL on collection elements and UDT fields cql3: parse per-element timestamps/TTLs in the selection layer cql3: add extended wire format for per-element timestamps and TTLs cql3: extend WRITETIME/TTL grammar to accept collection and UDT elements