/// <summary>
/// Split up a predicate into 2 parts - the pushdown and the non-pushdown predicate.
/// If the filter node has no external references *and* the "columns" parameter is null,
/// then the entire predicate can be pushed down
/// We then compute the set of valid column references - if the "columns" parameter
/// is non-null, this set is used. Otherwise, we get the definitions of the
/// input relop node of the filterOp, and use that.
/// We use this list of valid column references to identify which parts of the filter
/// predicate can be pushed down - only those parts of the predicate that do not
/// reference anything beyond these columns are considered for pushdown. The rest are
/// stuffed into the nonPushdownPredicate output parameter
/// </summary>
/// <param name="command"> Command object </param>
/// <param name="filterNode"> the FilterOp subtree </param>
/// <param name="columns"> (Optional) List of columns to consider for "pushdown" </param>
/// <param name="nonPushdownPredicateNode"> (output) Part of the predicate that cannot be pushed down </param>
/// <returns> part of the predicate that can be pushed down </returns>
private static Node GetPushdownPredicate(Command command, Node filterNode, VarVec columns, out Node nonPushdownPredicateNode)
{
var pushdownPredicateNode = filterNode.Child1;
nonPushdownPredicateNode = null;
var filterNodeInfo = command.GetExtendedNodeInfo(filterNode);
if (columns == null
&& filterNodeInfo.ExternalReferences.IsEmpty)
{
return pushdownPredicateNode;
}
if (columns == null)
{
var inputNodeInfo = command.GetExtendedNodeInfo(filterNode.Child0);
columns = inputNodeInfo.Definitions;
}
var predicate = new Predicate(command, pushdownPredicateNode);
Predicate nonPushdownPredicate;
predicate = predicate.GetSingleTablePredicates(columns, out nonPushdownPredicate);
pushdownPredicateNode = predicate.BuildAndTree();
nonPushdownPredicateNode = nonPushdownPredicate.BuildAndTree();
return pushdownPredicateNode;
}
