The Parsing Engine sends express requests to the AMP for data dictionary information from the DBC database that is not cached in memory.
Express requests avoid the multiple stages of the Parsing Engine, including resolving, syntax checking, permission checking, and optimization, that are required for user requests, such as SQL queries.
Express requests are directed straight to the AMP, which the RDBMS generates and distributes. This means that syntax and permission checks are unnecessary. Numerous Parsing Engine modules have the ability to produce express requests.
- The Optimizer will require access to statistics information.
- The Resolver will need to access DBC tables holding information about located objects.
- The module checking permissions will need to access the DBC tables containing information about permissions.
If the Parsing Engine requires access to the DBC and is not cached, Teradata will send an express request to the AMP(s). However, express requests can be troublesome if the AMP lacks worker tasks. At this point, they will be queued like other tasks in the AMP’s queue under workload type 01 or “continued work.”
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About the free bookDelays in parsing your query, or identifying missing PE modules, occur when AMP worker tasks are unavailable. This can be avoided by ensuring sufficient AMP worker tasks are available.
ParserExpReq counts express requests; it does not prove AWT starvation. Dictionary-cache misses can be normal. Correlate parsing delay, cache behavior and worker-task metrics before diagnosing a bottleneck.
Expediting Teradata Express Requests
The 14.10 update incorporates new functionalities that enable express requests to utilize the same mechanism employed for tactical workloads. On each AMP, certain worker tasks are reserved solely for workload types 08/09 associated with tactical queries. Reserving AWTs makes it possible to execute tactical queries even in a heavily loaded system, as these AWTs will be available unless fully occupied by other tactical workloads.
Reserved worker pools can reduce contention but may themselves be busy, so they do not guarantee zero queuing. Worktype numbers and reservation behavior here describe historical 14.10 behavior and require current platform documentation.
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Get the complete Teradata tuning book, free
Teradata Query Performance Tuning: The DWHPro Guide
By Roland Wenzlofsky & Artemios Vogiatzis
206 pages on statistics and demographics, access paths and indexes, query execution plans, and joins. Get the full PDF and practical Teradata updates from DWHPro.
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About the free book