1. A method for supporting imperative graphic queries on a relational database, comprising: parsing an imperative graphic query into operations of execution plans; translating operations of execution plans which are supported by the relational database into common table expression queries; executing the translated common table expression queries on the relational database; building a query tree based on the relational database according to translated queries; and reconstructing 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 indicate 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, and wherein the step of reconstructing the query tree to reduce the total estimated cost according to estimated cost for each node of the query tree includes: 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.