src/org/apache/pig/impl/logicalLayer/package.html - pig - Git at Google

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 <body>

 <p>
 The logical operators that represent a pig script and tools for manipulating
 those operators.  The logical layer contains the logical operators themselves,
 as well as validators that check the logical plan, an optimizer, and a general
 visitor utility for working with the logical plans.

 <h2> Design </h2>
 <p>
 Logical operators use the operator, plan, visitor, and optimizer framework
 provided by the {@link org.apache.pig.impl.plan} package.
 <p>
 Logical operators consist of both relational and expression operators.
 Relational operators work on an entire bag.  Expression operators work on an
 element of a tuple (which may also be a bag).  Due to Pig's nested data and
 execution model the distinction between relational and expression operators is
 not always clear.  And some operators such as LOProject function as both.
 <p>
 In a traditional data base system, a query execution plan is constructed from
 relational operators, such as project, filter, sort, aggregate, join.  Each of
 these may contain an expression tree, made up of expression operators.  For
 example, consider a SQL query <code>select a from T where a = 5;</code>.  The
 where clause would be represented by a filter operator with an expression tree
 for <code>a=5</code>.
 <p>
 Pig takes a similar approach, except that the operators contained inside of a
 relational operator may also be relational.  For example, a foreach statement
 that has a nested script, such as <code>foreach B { C = distinct $1; generate
 group, COUNT(C);}</code>.  This foreach needs to contain not just an
 expression tree but the distinct relational operator.  For this reason, Pig's
 relational operators do not contain expression trees.  Instead they contain
 one or more LogicalPlans themselves.  This allows Pig to arbitrarily nest
 the logical plan.  In this sense Pig is more similar to a traditional
 procedural language where certain statements (e.g. if, while) can contain any
 other statement in the language rather than being like SQL where the statement
 execution tends to be more linear.

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 </html>
	<html>
	<body>

	<p>
	The logical operators that represent a pig script and tools for manipulating
	those operators. The logical layer contains the logical operators themselves,
	as well as validators that check the logical plan, an optimizer, and a general
	visitor utility for working with the logical plans.

	<h2> Design </h2>
	<p>
	Logical operators use the operator, plan, visitor, and optimizer framework
	provided by the {@link org.apache.pig.impl.plan} package.
	<p>
	Logical operators consist of both relational and expression operators.
	Relational operators work on an entire bag. Expression operators work on an
	element of a tuple (which may also be a bag). Due to Pig's nested data and
	execution model the distinction between relational and expression operators is
	not always clear. And some operators such as LOProject function as both.
	<p>
	In a traditional data base system, a query execution plan is constructed from
	relational operators, such as project, filter, sort, aggregate, join. Each of
	these may contain an expression tree, made up of expression operators. For
	example, consider a SQL query <code>select a from T where a = 5;</code>. The
	where clause would be represented by a filter operator with an expression tree
	for <code>a=5</code>.
	<p>
	Pig takes a similar approach, except that the operators contained inside of a
	relational operator may also be relational. For example, a foreach statement
	that has a nested script, such as <code>foreach B { C = distinct $1; generate
	group, COUNT(C);}</code>. This foreach needs to contain not just an
	expression tree but the distinct relational operator. For this reason, Pig's
	relational operators do not contain expression trees. Instead they contain
	one or more LogicalPlans themselves. This allows Pig to arbitrarily nest
	the logical plan. In this sense Pig is more similar to a traditional
	procedural language where certain statements (e.g. if, while) can contain any
	other statement in the language rather than being like SQL where the statement
	execution tends to be more linear.

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	</html>