PostgreSQL Table in memory

You should not expect postgres processes to show large memory use, even if the whole database is cached in RAM.

That is because PostgreSQL relies on buffered reads from the operating system buffer cache. In simplified terms, when PostgreSQL does a read(), the OS looks to see whether the requested blocks are cached in the "free" RAM that it uses for disk cache. If the block is in cache, the OS returns it almost instantly. If the block is not in cache the OS reads it from disk, adds it to the disk cache, and returns the block. Subsequent reads will fetch it from the cache unless it's displaced from the cache by other blocks.

That means that if you have enough free memory to fit the whole database in "free" operating system memory, you won't tend to hit the disk for reads.

Depending on the OS, behaviour for disk writes may differ. Linux will write-back cache "dirty" buffers, and will still return blocks from cache even if they've been written to. It'll write these back to the disk lazily unless forced to write them immediately by an fsync() as Pg uses at COMMIT time. When it does that it marks the cached blocks clean, but doesn't flush them. I don't know how Windows behaves here.

The point is that PostgreSQL can be running entirely out of RAM with a 1GB database, even though no PostgreSQL process seems to be using much RAM. Having shared_buffers too high just leads to double-caching and can reduce the amount of RAM available for the OS to cache blocks.

It isn't easy to see exactly what's cached in RAM because Pg relies on the OS cache. That's why I referred you to pg_fincore.

If you're on Windows and this won't work, you really just have to rely on observing disk activity. Does performance monitor show lots of uncached disk reads? Does operating system memory monitoring show lots of memory used for disk cache in the OS?

Make sure that effective_cache_size correctly reflects the RAM used for disk cache. It will help PostgreSQL choose appropriate query plans.

You are making the assumption, without apparent evidence, that the query performance you are experiencing is explained by disk read delays, and that it can be improved by in-memory caching. This may not be the case at all. You need to look at explain analyze output and system performance metrics to see what's going on.