7. A computer-implemented method comprising: receiving a query for one or more data items stored in a database, wherein the query is associated with a query plan; determining whether to use at least one query operator that uses data having a format different from a storage model format of at least one of the data items stored in the database; and converting the format of the data used by the at least one query operator to another format that matches the storage model format of the at least one of the data items stored in the database, wherein the query plan is based on at least a syntax of the query and an order of clauses within the query, wherein a difference between the format of the data used by the at least one query operator and the storage model format of the at least one of the data items before conversion is one of the following: the format of the data used by the at least one query operator is a row-format and the storage model format of the at least one of the data items is a column-format; or the format of the data used by the at least one query operator is the column-format and the storage model of the at least one of the data items is the row-format, and wherein the receiving, the determining, and the converting are implemented on at least one processor.
9. The computer-implemented method of claim 7 , wherein the query plan is selected from a plurality of query plans for the query.