Steps to reproduce:

1. Execute sql query before problem time "EXPLAIN ANALYZE SELECT MAX(h.clock) FROM history_uint h WHERE h.itemid='10120751' AND h.clock>1737612045;"
2. Check execution time and kind of index
3. Execute sql query at problematic time "EXPLAIN ANALYZE SELECT MAX(h.clock) FROM history_uint h WHERE h.itemid='10120751' AND h.clock>1737612045;"
4. Check execution time and kind of index

Result:
Before problem time :
-Execution time <1ms
-Index  "_hyper_7_511_chunk_history_uint_1" btree (itemid, clock)
At problematic time :
-Execution time >5000ms
-Index  "_hyper_7_511_chunk_history_uint_new_clock_idx" btree (clock DESC)

Can you tell me why the SQL query behaves this way? And for what purposes is this index (*_chunk_history_uint_new_clock_idx" btree (clock DESC)) used in table 'history_uint'?