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query optimization

MongoDB’s New Matcher

May 28 • Posted 1 year ago

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MongoDB 2.5.0 (an unstable dev build) has a new implementation of the “Matcher”. The old Matcher is the bit of code in Mongo that takes a query and decides if a document matches a query expression. It also has to understand indexes so that it can do things like create a subsets of queries suitable for index covering. However, the structure of the Matcher code hasn’t changed significantly in more than four years and until this release, it lacked the ability to be easily extended. It was also structured in such a way that its knowledge could not be reused for query optimization. It was clearly ready for a rewrite.

The “New Matcher” in 2.5.0 is a total rewrite. It contains three separate pieces: an abstract syntax tree (hereafter ‘AST’) for expression match expressions, a parser from BSON into said AST, and a Matcher API layer that simulates the old Matcher interface while using all new internals. This new version is much easier to extend, easier to reason about, and will allow us to use the same structure for matching as for query analysis and rewriting.

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