When integrating information using a federated query planner it is useful to view the query plans to better understand how information is being accessed and processed, and to troubleshoot problems.

A query plan is a set of instructions created by a query engine for executing a command submitted by a user or application. The purpose of the query plan is to execute the user's query in as efficient a way as possible.

Getting a Query Plan

You can get a query plan any time you execute a command. The SQL options available are as follows:

SET SHOWPLAN [ON|DEBUG]- Returns the processing plan or the plan and the full planner debug log. See also the SET Statement.

With the above options, the query plan is available from the Statement object by casting to the org.teiid.jdbc.TeiidStatement interface or by using the "SHOW PLAN" statement.

statement.execute("set showplan on");
ResultSet rs = statement.executeQuery("select ...");
TeiidStatement tstatement = statement.unwrap(TeiidStatement.class);
PlanNode queryPlan = tstatement.getPlanDescription();
System.out.println(queryPlan);

The query plan is made available automatically in several of Teiid's tools.

Analyzing a Query Plan

Once a query plan has been obtained you will most commonly be looking for:

• Source pushdown -- what parts of the query that got pushed to each source
  • Ensure that any predicates especially against indexes are pushed

• Joins - as federated joins can be quite expensive
  • Join ordering - typically influenced by costing
  • Join criteria type mismatches.
  • Join algorithm used - merge, enhanced merge, nested loop, etc.

• Presence of federated optimizations, such as dependent joins.

• Ensure hints have the desired affects - see Hints and Options, hints in the FROM Clause, Subquery Optimization, and source hints below.

All of the above information can be determined from the processing plan. You will typically be interested in analyzing the textual form of the final processing plan. To understand why particular decisions are made for debugging or support you will want to obtain the full debug log which will contain the intermediate planning steps as well as annotations as to why specific pushdown decisions are made.

A query plan consists of a set of nodes organized in a tree structure. If you are executing a procedure or generating an XML document from an XML Document Model, the overall query plan will contain additional information related the surrounding procedural execution.

In a procedural context the ordering of child nodes implies the order of execution. In most other situation, child nodes may be executed in any order even in parallel. Only in specific optimizations, such as dependent join, will the children of a join execute serially.

Relational Execution Plans

Relational plans represent the processing plan that is composed of nodes representing building blocks of logical relational operations. Relational processing plans differ from logical debug relational plans in that they will contain additional operations and execution specifics that were chosen by the optimizer.

The nodes for a relational query plan are:

• Access - Access a source. A source query is sent to the connection factory associated with the source. [For a dependent join, this node is called Dependent Access.]

• Dependent Procedure Access - Access a stored procedure on a source using multiple sets of input values.

• Batched Update - Processes a set of updates as a batch.

• Project - Defines the columns returned from the node. This does not alter the number of records returned.

• Project Into - Like a normal project, but outputs rows into a target table.

• Insert Plan Execution - Similar to a project into, but executes a plan rather than a source query. Typically created when executing an insert into view with a query expression.

• Window Function Project - Like a normal project, but includes window functions.

• Select - Select is a criteria evaluation filter node (WHERE / HAVING).

• Join - Defines the join type, join criteria, and join strategy (merge or nested loop).

• Union All - There are no properties for this node, it just passes rows through from it's children. Depending upon other factors, such as if there is a transaction or the source query concurrency allowed, not all of the union children will execute in parallel.

• Sort - Defines the columns to sort on, the sort direction for each column, and whether to remove duplicates or not.

• Dup Remove - Removes duplicate rows. The processing uses a tree structure to detect duplicates so that results will effectively stream at the cost of IO operations.

• Grouping - Groups sets of rows into groups and evaluates aggregate functions.

• Null - A node that produces no rows. Usually replaces a Select node where the criteria is always false (and whatever tree is underneath). There are no properties for this node.

• Plan Execution - Executes another sub plan. Typically the sub plan will be a non-relational plan.

• Dependent Procedure Execution - Executes a sub plan using multiple sets of input values.

• Limit - Returns a specified number of rows, then stops processing. Also processes an offset if present.

• XML Table - Evaluates XMLTABLE. The debug plan will contain more information about the XQuery/XPath with regards to their optimization - see the XQuery section below or XQuery Optimization.

• Text Table - Evaluates TEXTTABLE

• Array Table - Evaluates ARRAYTABLE

• Object Table - Evaluates OBJECTTABLE

Node Statistics

Every node has a set of statistics that are output. These can be used to determine the amount of data flowing through the node.

In addition to node statistics, some nodes display cost estimates computed at the node.

The root node will display additional information.

Reading a Processor Plan

Source Hints

Teiid user and transformation queries can contain a meta source hint that can provide additional information to source queries. The source hint has the form:

/*+ sh[[ KEEP ALIASES]:'arg'] source-name[ KEEP ALIASES]:'arg1' ... */

• The source hint is expected to appear after the query (SELECT, INSERT, UPDATE, DELETE) keyword.
• Source hints may appear in any subquery or in views. All hints applicable to a given source query will be collected and pushed down together as a list. The order of the hints is not guaranteed.
• The sh arg is optional and is passed to all source queries via the ExecutionContext.getGeneralHints method. The additional args should have a source-name that matches the source name assigned to the translator in the VDB. If the source-name matches, the hint values will be supplied via the ExecutionContext.getSourceHints method. See the Developer's Guide for more on using an ExecutionContext.
• Each of the arg values has the form of a string literal - it must be surrounded in single quotes and a single quote can be escaped with another single quote. Only the Oracle translator does anything with source hints by default. The Oracle translator will use both the source hint and the general hint (in that order) if available to form an Oracle hint enclosed in /*+ ... */.
• If the KEEP ALIASES option is used either for the general hint or on the applicable source specific hint, then the table/view aliases from the user query and any nested views will be preserved in the push-down query. This is useful in situations where the source hint may need to reference aliases and the user does not wish to rely on the generated aliases (which can be seen in the query plan in the relevant source queries - see above). However in some situations this may result in an invalid source query if the preserved alias names are not valid for the source or result in a name collision. If the usage of KEEP ALIASES results in an error, the query could be modified by preventing view removal with the NO_UNNEST hint, the aliases modified, or the KEEP ALIASES option could be removed and the query plan used to determine the generated alias names.

SELECT /*+ sh:'general hint' */ ... 

SELECT /*+ sh KEEP ALIASES:'general hint' my-oracle:'oracle hint' */ ... 

Debug Plans

All Node Properties

Reading a Debug Plan

XQuery