OpenText Analytics Database 26.2.x – Query plans

Admin: Viewing query plans

Mon, 01 Jan 0001 00:00:00 +0000

You can obtain query plans in two ways:

The EXPLAIN statement outputs query plans in various text formats (see below).
Management Console provides a graphical interface for viewing query plans. For detailed information, see Working with query plans in MC.

You can also observe the real-time flow of data through a query plan by querying the system table QUERY_PLAN_PROFILES. For more information, see Profiling query plans.

EXPLAIN output options

By default, EXPLAIN output represents the query plan as a hierarchy, where each level, or path, represents a single database operation that the optimizer uses to execute a query. EXPLAIN output also appends DOT language source so you can display this output graphically with open source Graphviz tools.

EXPLAIN supports options for producing verbose and JSON output. You can also show the local query plans that are assigned to each node, which together comprise the total (global) query plan.

EXPLAIN also supports an ANNOTATED option. EXPLAIN ANNOTATED returns a query with embedded optimizer hints, which encapsulate the query plan for this query. For an example of usage, see Using optimizer-generated and custom directed queries together.

Admin: Query plan cost estimation

Mon, 01 Jan 0001 00:00:00 +0000

The query optimizer chooses a query plan based on cost estimates. The query optimizer uses information from a number of sources to develop potential plans and determine their relative costs. These include:

Number of table rows
Column statistics, including: number of distinct values (cardinality), minimum/maximum values, distribution of values, and disk space usage
Access path that is likely to require fewest I/O operations, and lowest CPU, memory, and network usage
Available eligible projections
Join options: join types (merge versus hash joins), join order
Query predicates
Data segmentation across cluster nodes

Many important optimizer decisions rely on statistics, which the query optimizer uses to determine the final plan to execute a query. Therefore, it is important that statistics be up to date. Without reasonably accurate statistics, the optimizer could choose a suboptimal plan, which might affect query performance.

OpenText™ Analytics Database provides hints about statistics in the query plan. See Query plan statistics.

Cost versus execution runtime

Although costs correlate to query runtime, they do not provide an estimate of actual runtime. For example, if the optimizer determines that Plan A costs twice as much as Plan B, it is likely that Plan A will require more time to run. However, this cost estimate does not necessarily indicate that Plan A will run twice as long as Plan B.

Also, plan costs for different queries are not directly comparable. For example, if the estimated cost of Plan X for query1 is greater than the cost of Plan Y for query2, it is not necessarily true that Plan X's runtime is greater than Plan Y's runtime.

Admin: Query plan information and structure

Mon, 01 Jan 0001 00:00:00 +0000

Depending on the query and database schema, EXPLAIN output includes the following information:

Tables referenced by the statement
Estimated costs
Estimated row cardinality
Path ID, an integer that links to error messages and profiling counters so you troubleshoot performance issues more easily. For more information, see Profiling query plans.
Data operations such as SORT, FILTER, LIMIT, and GROUP BY
Projections used
Information about statistics—for example, whether they are current or out of range
Algorithms chosen for operations into the query, such as HASH/MERGE or GROUPBY HASH/GROUPBY PIPELINED
Data redistribution (broadcast, segmentation) across cluster nodes

Example

In the EXPLAIN output that follows, the optimizer processes a query in three steps, where each step identified by a unique path ID:

0: Limit
1: Sort
2: Storage access and filter

Note

A storage access operation can scan more than the columns in the SELECT list— for example, columns referenced in WHERE clause.