Site Search system for e-commerce?

Nowadays every e-commerce platform is supplied with a search system. It's performance (as in relevant result to user input) usually lacks at it best and just plainly inefficient. Same pattern is observed both in opensource and commercial systems.

The problem is quite trivial - few thousands of products having name, description, categories, maker/brand, tags (keywords assigned to the product exposing relevant characteristics of it). Piecing everything together with assignment of proper weight coefficients (for sorting) is something tricky.

Are there any systems (preferably opensource but commercial would be fine as well) or approaches (methodology?) I am missing? Maybe good articles exposing how big companies like amazon maintain it's search systems? Anyone having experience of building search systems willing to share experience?

Appreciate any feedback.