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

A relational processing plan is created by the optimizer after the logical plan is manipulated by a series of rules. The application of rules is determined both by the query structure and by the rules themselves. The node structure of the debug plan resembles that of the processing plan, but the node types more logically represent SQL operations.

All Nodes

• ACCESS - a source access or plan execution.
• DUP_REMOVE - removes duplicate rows
• JOIN - a join (LEFT OUTER, FULL OUTER, INNER, CROSS, SEMI, etc.)
• PROJECT - a projection of tuple values
• SELECT - a filtering of tuples
• SORT - an ordering operation, which may be inserted to process other operations such as joins
• SOURCE - any logical source of tuples including an inline view, a source access, XMLTABLE, etc.
• GROUP - a grouping operation
• SET_OP - a set operation (UNION/INTERSECT/EXCEPT)
• NULL - a source of no tuples
• TUPLE_LIMIT - row offset / limit

User SQL statements after rewrite are converted into a cannonical plan form. The connonical plan form most closely resembles the initial SQL structure. For example, a SQL statement such as SELECT max(pm1.g1.e1) FROM pm1.g1 WHERE e2 = 1 creates a logical plan:

Project(groups=[anon_grp0], props={PROJECT_COLS=[anon_grp0.agg0 AS expr1]})
  Group(groups=[anon_grp0], props={SYMBOL_MAP={anon_grp0.agg0=MAX(pm1.g1.e1)}})
    Select(groups=[pm1.g1], props={SELECT_CRITERIA=e2 = 1})
      Source(groups=[pm1.g1])

Here the Source corresponds to the FROM clause, the Select correspondes to the WHERE clause, the Group corresponds to the implied grouping to create the max aggregate, and the Project corresponds to the SELECT clause.

Note that the affect of grouping generates what is effectively an inline view, anon_grp0, to handle the projection of values created by the gropuing.

Node Properties

Each node has a set of applicable properties that are typically shown on the node.

• Access Properties
  • ATOMIC_REQUEST - The final form of a source request
  • MODEL_ID - The metadata object for the target model/schema
  • PROCEDURE_CRITERIA/PROCEDURE_INPUTS/PROCEDURE_DEFAULTS - Used in planning procedureal relational queries
  • IS_MULTI_SOURCE - set to true when the node represents a multi-source access
  • SOURCE_NAME - used to track the multi-source source name
  • CONFORMED_SOURCES - tracks the set of conformed sources when the conformed extension metadata is used
  • SUB_PLAN/SUB_PLANS - used in multi-source planning
• SET_OPERATION/USE_ALL - defines the set operation (UNION/INTERSECT/EXCEPT) and if all rows or distinct rows are used.
• Join Properties
  • JOIN_CRITERIA - all join predicates
  • JOIN_TYPE - type of join (INNER, LEFT OUTER, etc.)
  • JOIN_STRATEGY - the algorithm to use (nested loop, merge, etc.)
  • LEFT_EXPRESSIONS - the expressions in equi-join predicates that originate from the left side of the join
  • RIGHT_EXPRESSIONS - the expressions in equi-join predicates that originate from the right side of the join
  • DEPENDENT_VALUE_SOURCE - set if a dependent join is used
  • NON_EQUI_JOIN_CRITERIA - non-equi join predicates
  • SORT_LEFT - if the left side needs sorted for join processing
  • SORT_RIGHT - if the right side needs sorted for join processing
  • IS_OPTIONAL - if the join is optional
  • IS_LEFT_DISTINCT - if the left side is distinct with respect to the equi join predicates
  • IS_RIGHT_DISTINCT - if the right side is distinct with respect to the equi join predicates
  • IS_SEMI_DEP - if the dependent join represents a semi-join
  • PRESERVE - if the preserve hint is preserving the join order
• Project Properties
  • PROJECT_COLS, // List <SingleElementSymbol>
  • INTO_GROUP, // GroupSymbol
  • HAS_WINDOW_FUNCTIONS, // Boolean
  • CONSTRAINT
• Select Properties
  • SELECT_CRITERIA, // Criteria
    IS_HAVING, // Boolean
    //phantom nodes represent the previous position of criteria that has been pushed across a source, group, or union node.
    //phantom nodes are used by RuleCopyCriteria and removed by RuleCleanCriteria.
    IS_PHANTOM, // Boolean
    IS_TEMPORARY, // Boolean
    IS_COPIED, // Boolean - used in CopyCriteria to mark which selects have already been copied
    IS_PUSHED, // true if this node has already been pushed
    IS_DEPENDENT_SET, // Boolean - only used with dependent joins
• Sort Properties
  • SORT_ORDER - the order by that defines the sort
    UNRELATED_SORT, // Boolean
    IS_DUP_REMOVAL, // Boolean
• Source Properties
  SYMBOL_MAP, // SymbolMap
  PARTITION_INFO, // Map<ElementSymbol, List<Set<Constant>>> - it will only be consistent in the initial stages of planning
  VIRTUAL_COMMAND, // Command
  MAKE_DEP, // Option.Makedep
  PROCESSOR_PLAN, // ProcessorPlan for non-relational sub plan
  NESTED_COMMAND, // Command for nested processor plan
  TABLE_FUNCTION, // Table Function
  CORRELATED_REFERENCES, // SymbolMap
  MAKE_NOT_DEP, // Boolean
  INLINE_VIEW, // If the source node represents an inline view
  NO_UNNEST,
  MAKE_IND,
• Group Properties
  GROUP_COLS, // List <Expression>
  ROLLUP, // Boolean
• Tuple Limit Properties
  MAX_TUPLE_LIMIT, // Expression that evaluates to the max number of tuples generated
  OFFSET_TUPLE_COUNT, // Expression that evaluates to the tuple offset of the starting tuple
  IS_IMPLICIT_LIMIT, // Boolean if the limit is created by the rewriter as part of a subquery optimization
  IS_NON_STRICT, // Boolean if the unordered limit should not be enforced strictly

SOURCE_HINT,

• General and Costing Properties
  OUTPUT_COLS, // List <SingleElementSymbol>
  EST_SET_SIZE, // Integer represents the estimated set size this node would produce for a sibling node as the independent node in a dependent join scenario
  EST_DEP_CARDINALITY, // Float value that represents the estimated cardinality (amount of rows) produced by this node as the dependent node in a dependent join scenario
  EST_DEP_JOIN_COST, // Float value that represents the estimated cost of a dependent join (the join strategy for this could be Nested Loop or Merge)
  EST_JOIN_COST, // Float value that represents the estimated cost of a merge join (the join strategy for this could be Nested Loop or Merge)
  EST_CARDINALITY, // Float represents the estimated cardinality (amount of rows) produced by this node
  EST_COL_STATS,
  EST_SELECTIVITY, // Float that represents the selectivity of a criteria node
  ACCESS_PATTERNS, // Collection <List <Object element ID> >
  ACCESS_PATTERN_USED, // List <Object element ID>
  REQUIRED_ACCESS_PATTERN_GROUPS,

Rules

Access Pattern Validation
Apply Security
Assign Output ElementSymbol
Calculate Cost
Choose Dependent
Choose Join Stategy
Clean Criteria
Collapse Source
Copy Criteria
Decompose Join
Implement Join Strategy
Merge Criteria
Merge Virtual
Place Access
Plan Joins
Plan Procedures
Plan Sorts
Plan Unions
Plan Aggregates
Push Limit
Push Non-Join Criteria
Push Select Criteria
Raise Access
Raise Null
Remove Optional Joins
Substitute Expressions
Validate Where All

Reading a Debug Plan

XQuery