public static HashSet <TableAlias> Gather(Expression expression)
{
var gatherer = new ReferencedAliasGatherer();
gatherer.Visit(expression);
return(gatherer.aliases);
}
private bool CanBeJoinCondition(Expression expression, HashSet <TableAlias> left, HashSet <TableAlias> right, HashSet <TableAlias> all)
{
// an expression is good if it has at least one reference to an alias from both left & right sets and does
// not have any additional references that are not in both left & right sets
var referenced = ReferencedAliasGatherer.Gather(expression);
var leftOkay = referenced.Intersect(left).Any();
var rightOkay = referenced.Intersect(right).Any();
var subset = referenced.IsSubsetOf(all);
return(leftOkay && rightOkay && subset);
}
/// <summary>
/// 访问 <see cref="JoinExpression"/>。
/// </summary>
/// <param name="join">要访问的表达式。</param>
/// <returns></returns>
protected override Expression VisitJoin(JoinExpression join)
{
join = (JoinExpression)base.VisitJoin(join);
if (join.JoinType == JoinType.CrossApply || join.JoinType == JoinType.OuterApply)
{
if (join.Right is TableExpression)
{
return(new JoinExpression(JoinType.CrossJoin, join.Left, join.Right, null));
}
else
{
var select = join.Right as SelectExpression;
// Only consider rewriting cross apply if
// 1) right side is a select
// 2) other than in the where clause in the right-side select, no left-side declared aliases are referenced
// 3) and has no behavior that would change semantics if the where clause is removed (like groups, aggregates, take, skip, etc).
// Note: it is best to attempt this after redundant subqueries have been removed.
if (select != null &&
select.Take == null &&
select.Skip == null &&
!AggregateChecker.HasAggregates(select) &&
(select.GroupBy == null || select.GroupBy.Count == 0))
{
var selectWithoutWhere = select.SetWhere(null);
var referencedAliases = ReferencedAliasGatherer.Gather(selectWithoutWhere);
var declaredAliases = DeclaredAliasGatherer.Gather(join.Left);
referencedAliases.IntersectWith(declaredAliases);
if (referencedAliases.Count == 0)
{
var where = select.Where;
select = selectWithoutWhere;
var pc = ColumnProjector.ProjectColumns(this.CanBeColumn, where, select.Columns, select.Alias, DeclaredAliasGatherer.Gather(select.From));
select = select.SetColumns(pc.Columns);
where = pc.Projector;
var jt = (where == null) ? JoinType.CrossJoin : (join.JoinType == JoinType.CrossApply ? JoinType.InnerJoin : JoinType.LeftOuter);
return(new JoinExpression(jt, join.Left, select, where));
}
}
}
}
return(join);
}