See QLC query interface for more details
The qlc module provides a query interface to Mnesia, ETS, Dets and other data structures that implement an iterator style traversal of objects.
MODULE
qlc
MODULE SUMMARY
Query Interface to Mnesia, ETS, Dets, etc
DESCRIPTION
The qlc module provides a query interface to Mnesia, ETS, Dets and other data structures that implement an iterator style traversal of objects.
Overview
The qlc module implements a query interface to QLC
tables. Typical QLC tables are ETS, Dets, and Mnesia
tables. There is also support for user defined tables, see the
Implementing a QLC
table section.
A query is stated using
Query List Comprehensions (QLCs). The answers to a
query are determined by data in QLC tables that fulfill the
constraints expressed by the QLCs of the query. QLCs are similar
to ordinary list comprehensions as described in the Erlang
Reference Manual and Programming Examples except that variables
introduced in patterns cannot be used in list expressions. In
fact, in the absence of optimizations and options such as
cache and unique (see below), every QLC free of
QLC tables evaluates to the same list of answers as the
identical ordinary list comprehension.
While ordinary list comprehensions evaluate to lists, calling qlc:q/1,2 returns a Query Handle. To obtain all the answers to a query, qlc:eval/1,2 should be called with the query handle as first argument. Query handles are essentially functional objects ("funs") created in the module calling q/1,2. As the funs refer to the module's code, one should be careful not to keep query handles too long if the module's code is to be replaced. Code replacement is described in the Erlang Reference Manual. The list of answers can also be traversed in chunks by use of a Query Cursor. Query cursors are created by calling qlc:cursor/1,2 with a query handle as first argument. Query cursors are essentially Erlang processes. One answer at a time is sent from the query cursor process to the process that created the cursor.
While ordinary list comprehensions evaluate to lists, calling qlc:q/1,2 returns a Query Handle. To obtain all the answers to a query, qlc:eval/1,2 should be called with the query handle as first argument. Query handles are essentially functional objects ("funs") created in the module calling q/1,2. As the funs refer to the module's code, one should be careful not to keep query handles too long if the module's code is to be replaced. Code replacement is described in the Erlang Reference Manual. The list of answers can also be traversed in chunks by use of a Query Cursor. Query cursors are created by calling qlc:cursor/1,2 with a query handle as first argument. Query cursors are essentially Erlang processes. One answer at a time is sent from the query cursor process to the process that created the cursor.
