Query method for a distributed database system and query apparatus

A computer program product for querying a distributed database system, the computer program product comprising a computer readable storage medium having program code embodied therewith, the program code readable and executable by a processor to perform a method comprising:

• parsing a query request;
• generating 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, 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 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 a query operation on the target database table according to the access plan;
• checking a consistent requirement of the query request;
• selecting, 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 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;
• selectin from the first candidate co set 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 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.

A computer system comprising:

• a processor, a computer readable memory, and a computer readable storage medium;
• first program instructions to parse a query request;
• second program instructions to generate 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;
• third program instructions to select, 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 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;
• fourth program instructions to execute a query operation in the target database table according to the access plan;
• fifth program instructions to check a consistent requirement of the query request;
• sixth program instructions to select 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 sue request being determined as having a consistency level that exceeds a first predefined level;
• seventh program instructions to select 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;
• eighth program instructions to select, from the first candidate copy set 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
• ninth program instructions to select, from the second candidate copy set 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;
• and wherein
• the first, second, third, fourth, fifth, sixth, seventh, eighth, and ninth program instructions are stored on the computer readable storage medium for execution by the processor via the computer readable memory.