10. A system for supporting imperative graphic queries on a relational database, comprising: means configured to parse an imperative graphic query into operations of execution plans; means configured to translate operations of execution plans which are supported by the relational database into common table expression queries; means configured to execute the translated common table expression queries on the relational database; means configured to build a query tree based on the relational database according to translated queries; and means configured to reconstruct the query tree to reduce a total estimated cost according to estimated cost for each node of the query tree, wherein the query tree is a Left deep join tree, each connection father node having two child nodes in the left deep join tree indicates a connection operation between two relational data sets, each non-connection father node having only one child node and this child node indicates a selection or projection operation for one relational data set, respective nodes of the left deep join tree are numbered in post order traversal, and a leftmost child node has a least number, wherein the means configured to reconstruct the query tree to reduce the total estimated cost according to estimated cost for each node of the query tree is configured to: carry out a process on each connection father node sequentially in a number order with the connection father node with the least number as a start, wherein the process on one connection father node includes: estimating the cost for two child trees connected with the connection father node and the cost for obtaining the connection father node by connecting the two child trees; and determining whether a left child tree and a right child tree are exchangeable by: estimating the cost for the connection father node after exchanging the left and right child trees; and if the cost after exchanging the left and right child trees is reduced, then exchanging the left and right child trees to rewrite a connection order of the connection father node.
10. A system for supporting imperative graphic queries on a relational database, comprising: means configured to parse an imperative graphic query into operations of execution plans; means configured to translate operations of execution plans which are supported by the relational database into common table expression queries; means configured to execute the translated common table expression queries on the relational database; means configured to build a query tree based on the relational database according to translated queries; and means configured to reconstruct the query tree to reduce a total estimated cost according to estimated cost for each node of the query tree, wherein the query tree is a Left deep join tree, each connection father node having two child nodes in the left deep join tree indicates a connection operation between two relational data sets, each non-connection father node having only one child node and this child node indicates a selection or projection operation for one relational data set, respective nodes of the left deep join tree are numbered in post order traversal, and a leftmost child node has a least number, wherein the means configured to reconstruct the query tree to reduce the total estimated cost according to estimated cost for each node of the query tree is configured to: carry out a process on each connection father node sequentially in a number order with the connection father node with the least number as a start, wherein the process on one connection father node includes: estimating the cost for two child trees connected with the connection father node and the cost for obtaining the connection father node by connecting the two child trees; and determining whether a left child tree and a right child tree are exchangeable by: estimating the cost for the connection father node after exchanging the left and right child trees; and if the cost after exchanging the left and right child trees is reduced, then exchanging the left and right child trees to rewrite a connection order of the connection father node. 11. The system according to claim 10 , further comprising: means configured to translate operations of execution plans which are not supported by the relational database into user-defined function queries; and means configured to execute translated user-defined function queries on the relational database.