huge-table | Table component to handle huge sets | Grid library

Which files from the target directory are actually required by the executable

None of them, other than the executable itself. By default, Rust produces statically-linked binaries.

The other files are merely build artifacts maintained by Cargo in order to make rebuilding your code more efficient. They include things like your dependencies.

A non-exhaustive sampling of some of the files you might find:

• *.d — Makefile-compatible dependency lists
• *.rlib — Rust library files. Contain the compiled code of a dependency
• build — Directories for build scripts to use as scratch space
• deps — Your compiled dependencies
• examples — Binaries from the examples directory
• incremental — A directory for the incremental compilation cache
• *-{hash} — Binaries from cargo test
• executables — Your target binaries

Some of this is documented in the Cargo source code.

The Index-only scans wiki says

It is important to realise that the planner is concerned with minimising the total cost of the query. With databases, the cost of I/O typically dominates. For that reason, "count(*) without any predicate" queries will only use an index-only scan if the index is significantly smaller than its table. This typically only happens when the table's row width is much wider than some indexes'.

and

Index-only scans are only used when the planner surmises that that will reduce the total amount of I/O required, according to its imperfect cost-based modelling. This all heavily depends on visibility of tuples, if an index would be used anyway (i.e. how selective a predicate is, etc), and if there is actually an index available that could be used by an index-only scan in principle

Accordingly, your index is not considered "significantly smaller" and the entire dataset is to be read, which leads the planner in using a seq scan