Tree::Predicate 0.03 NAME	Tree::Predicate - a tree of predicate logic (e.g. an SQL predicate) SYNOPSIS	use Tree::Predicate qw(:logical);	my $left_branch = OR('a', 'b');	my $right_branch = OR('c', 'd');	my $tree = AND($left_branch, $right_branch);	my $sql = $tree->as_string;	my @subtrees = $tree->split;	for (@subtrees) {	print $_->as_string;	} DESCRIPTION	Improving static queries for optimal performance is almost as old	as SQL itself. A relatively recent discussion (on which this module	is based) can be found in the book "High Performance MySQL" from	O'Reilly. One technique for improving queries relies on the way	that MySQL chooses which indexes to use. Prior to MySQL 5.0, a query	such as:	SELECT * FROM foo WHERE a = 1 OR b = 2	would result in a full-table scan, regardless of whether foo is indexed	on a or b. For MySQL 5.0 or later, a combination index would be used	for that query.	But for this query:	SELECT * FROM foo WHERE c IN ('foo', 'bar') AND (a = 1 OR b = 2)	both before-5.0 and after-5.0 will use an index on c before considering	a merged index of a/b. For such queries, a significant improvement	is available by rewriting the query as:	SELECT * FROM foo WHERE c IN ('foo', 'bar') AND a = 1	UNION	SELECT * FROM foo WHERE c IN ('foo', 'bar') AND b = 2	Even if we are using placeholders, it is straight-forward to rewrite	any static query in whatever form performs best. What about queries	that are created dynamically, though?	This is where Tree::Predicate comes in. Construct a tree with its	logical operators (AND/OR/NOT). The "split" method will return a	list of subtrees that can be used as predicates in a series of	UNION queries that are logically equivalent to the original predicate.	The subtrees can be used to construct a complete query or can be used	to query a database successively so that previous results can be used to	modify subsequent queries. Suppose we have a tree $tree that is	being used to query table items:	my @ids;	for my $subtree ($tree->split) {	if (@ids) {	my $ids = join ',' => @ids;	my $phrase = "NOT IN ($ids)";	my $new_subtree = AND($phrase, $subtree);	my $sql =	"SELECT id FROM items WHERE " . $new_subtree->as_string;	my $aryref = $dbh->selectcol_arrayref($sql);	push @ids, @$aryref;	} else {	my $sql =	"SELECT id FROM items WHERE " . $subtree->as_string;	my $aryref = $dbh->selectcol_arrayref($sql);	@ids = @$aryref;	}	}	This is just an example, of course. Actual implementations will	probably be more sophisticated. CAVEATS	Particularly with slower predicate atoms, such as REGEXP and LIKE, it is	possible to turn a slow query into a VERY slow query by splitting its	predicate into several. You will probably want a heuristic that governs	when you try to improve the dynamically-generate query or whether you	just give the whole mess to the database to sort out. FUTURE DIRECTIONS	I have several ideas for future directions for this module.	One is to allow associating anticipated selectivity and cost information	with an atom so that the module can order subtrees accordingly.	(Obviously, we should choose to execute predicates that can find as much	of the total result as cheaply as possible.)	Two related ideas would entail giving the tree a database handler, plus	other information, so that it can compute its own result. This would	allow the short-circuit emulation above to become internal to the module.	Additionally, tables that are joined for the purpose of evaluating one	part of the predicate could be joined when that predicate is part of	the subtree being evaluated. Some parts of a predicate could be	stored elsewhere, such as in a cache, which could avoid some database	activity. INSTALLATION To install this module, run the following commands:	perl Makefile.PL	make	make test	make install SUPPORT AND DOCUMENTATION After installing, you can find documentation for this module with the perldoc command. perldoc Tree::Predicate You can also look for information at: RT, CPAN's request tracker http://rt.cpan.org/NoAuth/Bugs.html?Dist=Tree-Predicate AnnoCPAN, Annotated CPAN documentation http://annocpan.org/dist/Tree-Predicate CPAN Ratings http://cpanratings.perl.org/d/Tree-Predicate Search CPAN http://search.cpan.org/dist/Tree-Predicate/ COPYRIGHT AND LICENSE Copyright (C) 2010 Yahoo!, Inc. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.