Reads which need sstable index were computing
column_values_fixed_lengths each time. This showed up in perf profile
for a sstable-read heavy workload, and amounted to about 1-2% of time.
Computing it involves type name parsing.
Avoid by using cached per-sstable mapping. There is already
sstable::_column_translation which can be used for this. It caches the
mapping for the least-recently used schema. Since the cursor uses the
mapping only for primary key columns, which are stable, any schema
will do, so we can use the last _column_translation. We only need to
make sure that it's always armed, so sstable loading is augmented with
arming with sstable's schema.
Also, fixes a potential use-after-free on schema in column_translation.
Closesscylladb/scylladb#21347
* github.com:scylladb/scylladb:
sstables: Fix potential use-after-free on column_translation::column_info::name
sstables: Avoid computing column_values_fixed_lengths on each read
05246e123d