+ <blockquote>the given path denotes a structured metadata whose components
+ are found in the fields of the given table. For example:<br>
+ <br>
+ refobj <- refObj<br>
+ <br>
+ tells that the path <b>/"refObj"</b> denotes a structured metadata whose
+ components are found in the fields of the table "refobj"; <br>
+ </blockquote>
+
+ <ul>
+ <li><table_name> "<+" [ <path_component> ]*</li>
+
+ </ul>
+
+ <blockquote>same as the previous but tells that this is a default correspondence;
+ <br>
+ </blockquote>
+
+ <ul>
+ <li><virtual_table_name> "->" <concrete_table_name></li>
+
+ </ul>
+
+ <blockquote>defines a correspondence between a virtual table name an
+ a concrete table name. All the <table_name> entries represent virtual
+ table names that are mapped to concrete table names using the identity function
+ unless a particular mapping is defined for them using the above construction.
+ This mechanism allows to define several set of metadata on the same database
+ table as in:<br>
+ <br>
+ refobj source
+ <-<br>
+ refobj h_occurrence <-
+ refObj h:occurrence<br>
+ backpointer source
+ <- backPointer h:occurrence<br>
+ backpointer h_occurrence <-<br>
+ backpointer
+ -> refobj<br>
+ </blockquote>
+
+ <blockquote>which defines four path accessing two virtual tables ("refobj"
+ and "backpointer") and then maps these tables in a single concrete table;<br>
+ </blockquote>
+
+ <ul>
+ <li>"->" <br>
+ </li>
+
+ </ul>
+
+ <blockquote>a line like this must end the map file. <br>
+ </blockquote>
+ Here you can find the <a
+ href="http://www.cs.unibo.it/cgi-bin/cvsweb/helm/mathql_db_map.txt">current
+ version of PostgreSQL database map for HELM</a>.<br>
+ <br>
+ <b>How does the interpreter use the map?</b> The map file is read
+during the interpreter initialization process from the file pointed by the
+MATHQL_DB_MAP environment variable and is used during the execution of each
+ <i>property</i> operation in the issued queries.When executing a <i>property</i>
+operation, the interpreter uses the map to find the smallest set of database
+tables containing the information required by the given access paths and then
+queries these tables to obtain the wanted information. <br>
+ </td>
+ </tr>
+
+ </tbody>