Query Capabilities

Some data sources such as Web sites or systems that offer a Web Service interface do not allow any query to their data, instead they present interfaces with limited query capabilities (e.g. HTML forms in the case of a Web site and a list of parameters in the case of a Web Service operation). For this reason, Virtual DataPort allows the administrator to explicitly specify the query capabilities of the base views. In addition, the execution engine automatically calculates the query capabilities of the derived views according to the capabilities of its base views.

The “Search methods” tab of the view displays the list of search methods of the view. You can modify the search methods of the base views, not those of derived views.

The query capabilities are expressed using a list of search methods. Each search method specifies a different way to query the view (or base relation). See the next section for a detailed description of how to interpret search methods of a view (or base relation).

When you create a base view using the administration tool, the tool automatically defines the search methods of the view. While almost always, these search methods automatically created do not need to be modified, this may not always be the case, particularly in sources of the type Web and Web Service.

Once in the advanced configuration view of a base view, the generated search methods can be changed easily. As can be seen in the figure, a search method specifies for each attribute of the view (see the subsection below for more details):

Search methods of a base view

Search methods of a base view

  • Its data type.
  • A list of operators for which the field can be queried. “any” indicates any operator. New operators may be added for the attribute using the drop-down menu in its “Operators” column. Click on image0, beside an operator, to remove it.
  • Whether it is obligatory (OBL), optional (OPT) or not supported (NOS) in the queries to the view.
  • The multiplicity supported for the attribute in queries to the view.
  • A list of the possible values for the attribute that may appear in a condition of a valid query to the view. If no value is specified, it is assumed that every value of the attribute data type is valid. To add new values simply enter them into the box that appears in the “Values” column of the attribute and press the button that appears on the right. Click on image0, beside a value, to remove it.
  • Whether the attribute appears in the response of the queries made to the view.

There is no graphical support to add or delete search methods from a base view; just to modify the existing ones.

Alternative Sources

When the data of a table of a database is replicated in other databases, you should configure this base view to indicate the databases where the same data can be located.

The section Selecting the Most Optimal Source When the Data Is Replicated in Several Sources explains in more detail why defining where the data of a base view is replicated can increase the performance of the queries involving this base view.

This feature is only available for JDBC base views.

To define additional sources for the same base view, do the following:

  1. Create a JDBC data source for each database in which the table of this base view is replicated. Each data source has to point to one of the databases. If you already have data sources that point to these databases, you do not need to create them again.
  2. Go to the Search methods tab of the Options dialog of the view.
  3. Click on image1, next to Alternative wrappers.
Add alternative sources for a base view: selecting an alternative table

Add alternative sources for a base view: selecting an alternative table

  1. In the wizard “Add alternative source”, on the JDBC data source database list, select the database where you have created one of the alternative data sources.
  2. In the JDBC data source list, select the data source that points to the database with the table.
  3. Below, the Tool will populate a tree of the schemas of this data source. In this dialog, select the table/view that contains a copy of the data of the base view you are currently editing. In this tree, you can only select one table or view.
  4. Click Create selected. The Tool will display a dialog like the one below.
Add alternative sources for a base view: defining the mappings

Add alternative sources for a base view: defining the mappings

  1. In this dialog you can define mappings between the fields of the base view and the table of the database. This is useful if the names of the tables in the alternative source are different from the fields of the base view.

    The base view automatically links the fields of the base view with the fields of the table when their name is the same.

  2. After clicking Ok, there will be a new entry in the “Alternative wrappers” section of the search methods tab.

Add alternative sources for a base view

Add alternative sources for a base view

  1. Repeat these steps for each database that contains a replica of the table of this base view.
  2. For each alternative wrapper, you can define its i18n, which affects how the timestamp values are treated. However the default value (“DEFAULT”) is almost always the correct one.

Search Methods

Each base relation explicitly describes its query capabilities through what are known as search methods. If a relation has no search method, then no query can be made to it.

Each search method is comprised of a series of 5-tuples. Each 5-tuple represents a restriction that a specific query should comply with to be executed on the source using this search method. In this sense, a search method that has no 5-tuple allows every query (this is the case of sources that have no limitations in their query capabilities such as, for example, conventional relational databases).

The format of a 5-tuple is (attribute, operators, obligatoriness, multiplicity, possible_values) where:

  • attribute is an attribute of the relation.
  • operators is the group of operators that can be used in conditions with this attribute in the queries to this view and with this search method. “ANY” represents any operator allowed by the attribute data type.
  • obligatoriness can have three values: “OBL” indicates that the attribute should mandatorily appear in any query to the source. “OPT” indicates that the attribute may or may not appear in the query (it is optional) and “NOS” indicates that the queries for this attribute are not allowed in the source.
  • multiplicity indicates how many values can be included in the source query for the given attribute and operator. If it is not possible to make queries for this attribute (“NOS” value in the obligatoriness field), the value is necessarily 0. “ANY” indicates that a query, for any given attribute and operator, could have any number of values.
  • possible_values is the list of values that can be used to query the attribute. If the value “ANY” is contained in it, this means that the search range is not limited (within the range associated with the attribute data type), and the attribute can be queried about any value. If the obligatoriness field is set in the 5-tuple to the “NOS” value, then it necessarily takes the value of an empty set.

Example: Consider the example of a virtual bookshop on the Internet offering the search form of shown in Search form for a bookshop.

Search form for a bookshop

Search form for a bookshop

The form requires the user to specify a value for the TITLE attribute and gives the option to set a value for the AUTHOR attribute and for the FORMAT attribute (restricted to a group of values). Searches by title and author are searches by keyword (operator like). A search by exact phrase (operator =) is indicated by selecting the box next to the search box of the field. For each attribute, a search can specify one value only. In addition to the fields TITLE, AUTHOR and FORMAT, the shop returns as output a PRICE attribute, which cannot be queried using the form.

Let us model this source as a relation R={TITLE, AUTHOR, FORMAT, PRICE} with a search method containing the 5-tuples shown in Search method for a bookshop.

Search method for a bookshop
(TITLE,{like,=}, OBL, 1, Any)
(AUTHOR, {like,=}, OPT, 1, Any)
(FORMAT, {=}, OPT, 1,
{'All formats', 'Hardcover', 'eBooks', 'Paperbacks'})
(PRICE, {}, NOS, 0, {})

Note that the first 5-tuple has the value {like, =} in the OPERATORS field and OBL in the OBLIGATORINESS field, this does not mean that it is obligatory to query the TITLE attribute with both operators, but that it is obligatory to query it at least with one of them. In order to have the TITLE attribute appear obligatorily in the query with both operators (this is not possible in the form in the example), this should be done with two different 5-tuples for the TITLE attribute, one for each operator:

{(TITLE, {like}, OBL, 1, ANY) (TITLE, {=}, OBL, 1, ANY)}

Thus, as can be seen, when you want to differentiate the treatment of a specific attribute according to the operator with which it is used, more than one 5-tuple can exist for each attribute.

When views or relations of the global schema are created, Virtual DataPort can automatically calculate its search methods from those of the sources and the expression used to define the view. This allows the system to know a priori whether a specific query can be responded to. Therefore, for derived views, Virtual DataPort allows the query capacities to be consulted, but they cannot be modified directly.