1. A computer-implemented method of generating a decision tree model comprising: generating a plurality of bitmaps in the database system, the bitmaps generated from data stored in a database table in the database system, the database table comprising a plurality of rows of data, the plurality of bitmaps comprising a bitmap for each unique value of each predictor and target and indicating whether or not that unique value of each predictor and target is present in each row of the database table; intersecting each split node bitmap with each predictor bitmap and with each target bitmap to form intersected bitmaps; counting bits of each intersected bitmap to generate a count of predictor-target pairs; determining a splitter value for the data in the database table using the counts of the predictor-target pairs so as to split the data in the database table into a plurality of child nodes, each child node comprising a portion of the data in the database table; generating child bitmaps for the data in each child node; recursively generating a split node bitmap for each child node by intersecting a parent node bitmap and a bitmap of a predictor that satisfies a condition of the child node; intersecting each child node bitmap with each predictor bitmap and with each target bitmap to form intersected bitmaps; and counting bits of each intersected bitmap to generate a count of predictor-target pairs; whereby a decision tree model is formed; wherein the bitmap of the predictor that satisfies the condition of the child node is generated by: if the node is a root node, generating a bitmap by ORing each bitmap for each value of the predictor that satisfies a condition of the node split, to form a single bitmap for all values of the predictor that satisfy the condition of the node split; and if the node is below the root node, generating a bitmap by ORing each bitmap for each value of the predictor that satisfies a condition of the node split, to form a single bitmap for all values of the predictor that satisfy the condition of the node split and ANDing the single bitmap with a bitmap for a node above the node.