read_indexes was one of the first functions coded in the sstable read path. At
the time, I made the (now so obviously) wrong decision to code it generic
enough so that we could specify the number of items to be read, instead of an
upper bound in the file.
The main reason for that, was that without the Summary, we have no way to know
where to stop reading, and the Summary is a relatively new addition to the C*
codebase: while I didn't really check when it got in, the code is full of tests
for its presence.
That turned out to be totally useless: we always read the indexes with the help
of the Summary. While the Summary is a relatively new addition to C*, it is
present in all version we aim to support. Meaning that reads without the
Summary will never happen in our codebase.
Even if, in the future, we happen to ditch the Summary file, we are very likely
to do so in favor of some other structure that also allows us to manipulate precise
borders in the Index.
The code as it is, however, would not be too big of a problem if that wasn't
causing us performance problems. But it is, and the majority of it is caused by
the fact that our underlying read_indexes do not know in advance how many bytes
to read, forcing us to do an element-per-element read.
It's time for a change.
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
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