Elimination of query fragment duplication in complex database queries

A database engine receives a database query from a client. The database engine parses the database query to build a query operator tree that includes a plurality of query operators. The database engine performs one or more optimization passes on the query operator tree, including a deduplication optimization pass, to form an optimized execution plan. The deduplication optimization pass includes: creating a list of query operators via a first traversal of the query operator tree, determining a first query operator that is equivalent to a second query operator, based on a hash map, via a second traversal of the query operator tree, and substituting, via a third traversal of the query operator tree, the second query operator with a tree node that links to the first query operator. The database engine executes the optimized execution plan to retrieve a result set from the database, and returns the result set.