1. A method for querying a distributed database system, the method comprising: parsing, by one or more processors, a query request; generating, by one or more processors, an access plan for the query request, wherein the access plan specifies therein a database table related to the query request, and wherein a copy of the database table is stored in multiple database devices; selecting, by one or more processors and based on status information of each copy of the database table, one copy of the database table from a plurality of copies of the database table as a target database table, wherein the status information of each of the plurality of copies of the database table comprises: consistent status information indicating whether a particular copy of the database table is consistent with other copies of the database table; availability status information indicating an availability of a particular copy of the database table; and load status information indicating workload of a database device in which a particular copy of the database table is stored; executing, by one or more processors, a query operation on the target database table according to the access plan; checking, by one or more processors, a consistent requirement of the query request; selecting, by one or more processors and based on the consistent status information of each of the plurality of copies, at least one copy of the database table whose consistent status information indicates that said at least one copy is consistent with other copies of the database table to form a first candidate copy set, wherein the first candidate copy set is selected in response to the consistent requirement of the query request being determined as having a consistency level that exceeds a first predefined level; selecting, by one or more processors, the plurality of copies to form a first candidate copy set without considering the consistent status information if the consistent requirement of the query request has a consistency level that is below a second predefined level; selecting from the first candidate copy set, by one or more processors and based on the availability status information of each copy of the database table in the first candidate copy set, at least one copy of the database table whose availability status information indicates that said at least one copy of the database table is available to form a second candidate copy set; and selecting from the second candidate copy set, by one or more processors and based on the load status information of each copy in the second candidate copy set, a copy of the database table, whose load status information indicates that the database device that stores the copy has a lowest workload compared to other database devices that are soring the copy of the database table, as the target database table.