Important Classes and Interfaces within the org.pentaho.pms.mql package:

MQL Query Model

See the document Pentaho Metadata MQL Schema for more details on the XML configuration.

Important Algorithms

Deserialization of MQL XML

The MQLQueryImpl class provides the functionality to parse MQL XML. First an XML DOM is generated from a String, and then that DOM is traversed via the fromXML() methods. The dom is broken into five segments.

The first section includes the parsing of domain_type, domain_id, and model_id. These are all required.

The options element contains customized options that impact the generation of the SQL. The only option defined within MQLQueryImpl is disable_distinct.

Selection elements contain the business column selections for the query in the column child element. Selection objects are created for each xml element.

Constraint elements contain an open formula expression in the condition child element, and an operator definition as a child element. A new WhereCondition object is generated for each constraint.

Order elements contain direction, view_id, and column_id child elements. direction may equal "asc" or "desc", and the view_id and column_id elements must resolve to an existing business column. An OrderBy object is generated for each order element.

Serialization of MQL XML

The MQLQueryImpl class provides functionality to generate MQL XML based on it's object model of selections, constraints, orders, and options. The getXML() call first generates an XML DOM, and then prints that out to a string.

SQL Generation

The SQLGeneration class combined with the PMSFormula class contain the code necessary for generating SQL based on the MQL model. First, an optimal join path is determined that contains all selected and referenced business tables. From there, the SQL is generated in seven generation steps. These generation steps include Select, From, Join, Where, Group By, Having, and Order By.

Join Path Algorithm

The getShortestPathBetween() method determines the shortest path between the list of included tables within the MQL Query.

The algorithm first determines if there is an existing path between all selected tables. If not, the algorithm continues to add new tables to the list until a path is discovered. If more than one path is available with a certain number of tables, the algorithm uses the relative size values if specified to determine which path to traverse in the SQL Join.

Select

The generateSelect() method traverses the set of selections and renders those selections to the SQL string buffer. This method determines the SQL column aliases. It also calls getBusinessColumnSQL() which renders each individual business column in three different ways. Either as an MQL Formula, an aggregate function, or as a standard SQL column.

From

The generateFrom() method traverses the set of included business tables and renders those tables to the SQL string buffer.

Joins

The generateJoins() method traverses the set of included RelationMetas and renders them to the SQL string buffer via the getJoin() method

Where

The generateWhere() method renders all WhereCondition's that are not part of an aggregate column.

Group By

The generateGroupBy() method renders the non aggregate columns in this section, creating the correct SQL for group by statements.

Having

The generateHaving() method renders all the Where Conditions of aggregate columns.

Order By

The generateOrderBy() method renders all the Order By business columns

PMSFormula Class

The PMSFormula class is used in various contexts while generating SQL from MQL. These contexts include Business Column Exact Formula rendering, Relation Complex Join rendering, and Constraint rendering.

Business Column Exact Formula Rendering

Used within the SQLGeneration.getBusinessColumnSQL() method when a business column is configured as "exact", the formula specified as part of the business column is parsed as an open formula and then rendered as SQL.

Relation Complex Join Rendering

Used within the SQLGeneration.getJoin() method when a Relation is set to complex, the complex join string is parsed as open formula and rendered to SQL.

Constraint Rendering

Constraints are rendered using PMSFormula. Each WhereCondition class contains a PMSFormula object which manages the SQL rendering. Constraints are parsed and resolved before SQL generation, to determine which business tables are referenced. This is an important step so that the Join algorithm contains the correct set of tables.

 Extending MQL 

Specifying your own MQLQuery class

The default MQLQuery behavior may be extended or replaced.  To accomplish this:

Extending MQLQueryImpl

To easily add modifications or additional information to the MQL XML syntax, one must extend MQLQueryImpl, SQLGenerator and PMSFormula.  The MQL Query Model can be extended by overriding specific methods within these classes.  In the org.pentaho.pms.example package, an AdvancedMQLQuery class is available that demonstrates the extension of Selections XML parsing.