Long-term index caching in the global cache, as introduced in 4.6, is a major
pessimization for workloads where accesses to the index are (spacially) sparse.
We want to have a way to disable it for the affected workloads.
There is already infrastructure in place for disabling it for BYPASS CACHE
queries. One way of solving the issue is hijacking that infrastructure.
This patch adds a global flag (and a corresponding CLI option) which controls
index caching. Setting the flag to `false` causes all index reads to behave
like they would in BYPASS CACHE queries.
Consequences of this choice:
- The per-SSTable partition_index_cache is unused. Every index_reader has
its own, and they die together. Independent reads can no longer reuse the
work of other reads which hit the same index pages. This is not crucial,
since partition accesses have no (natural) spatial locality. Note that
the original reason for partition_index_cache -- the ability to share
reads for the lower and upper bound of the query -- is unaffected.
- The per-SSTable cached_file is unused. Every index_reader has its own
(uncached) input stream from the index file, and every
bsearch_clustered_cursor has its own cached_file, which dies together with
the cursor. Note that the cursor still can perform its binary search with
caching. However, it won't be able to reuse the file pages read by
index_reader. In particular, if the promoted index is small, and fits inside
the same file page as its index_entry, that page will be re-read.
It can also happen that index_reader will read the same index file page
multiple times. When the summary is so dense that multiple index pages fit in
one index file page, advancing the upper bound, which reads the next index
page, will read the same index file page. Since summary:disk ratio is 1:2000,
this is expected to happen for partitions with size greater than 2000
partition keys.
Fixes#11202
cdb3e71045