The LOCAL option available on many functions (like JOIN, SORT, etc.) and the LOCAL() and NOLOCAL() functions control whether the graphs running on a particular node access all parts of a file/index or only those associated with the particular node (LOCAL). Often within an ALLNODES() context you only want to access local index parts from a single node because each node is independently processing its associated parts. Specifying that an index read or a keyed JOIN is LOCAL means that only the local part is used on each node. A local read of a single part INDEX will only be evaluated on the first agent node (or the farmer node if not within an ALLNODES)
Local evaluation can be specified in two ways:
1) As a dataset operation:
LOCAL(MyIndex)(myField = searchField)2) As an option on the operation:
JOIN(... ,LOCAL) FETCH(... ,LOCAL)
The LOCAL(dataset) function causes every operation on the dataset to access the file/key locally. For example,
LOCAL(JOIN(index1, index2,...))
will read index1 and index2 locally. This rule is recursively applied until you reach one of the following:
Use of the NOLOCAL() function
A non-local attribute--the operation stays non-local, but children are still marked as local as necessary
A GLOBAL() or THISNODE() or workflow operation--since they will be evaluated in a different context
Use of the ALLNODES() function (as in a nested child query)
Note that:
JOIN(x, LOCAL(index1)...) is treated the same as JOIN(x, index1, ..., local).
LOCAL is also supported as an option on an INDEX, but the LOCAL() function is preferred, because it generally depends on the context an index is used in whether or not access to it should be local or not.
A non-local attribute is supported everywhere that a LOCAL attribute is allowed - to override an enclosing LOCAL() function.
The use of LOCAL to indicate that dataset/key access is local does not conflict with its use to control coordination of an operation with other nodes, because there is no operation that potentially co-ordinates with other nodes and also accesses indexes or datasets.