IBM Data and AI Ideas Portal for Customers


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Status Not under consideration
Workspace Informix
Components Informix Server
Created by Guest
Created on Mar 7, 2023

RELATED IDEAS

UPDATE STATISTICS MEDIUM/HIGH with PDQ should favour use of indices for sort when more than one column is specified

Currently (in my testing) any UPDATE STATISTICS MEDIUM/HIGH statement with PDQ specifying more than one column will not use indices for sort and will instead scan the table, write to temporary files and sort. This is true even when all columns specified are lead columns in an index.

This makes use of PDQ 2-3x slower than the non-PDQ case where indices are used for sort where available. Not only this but much more I/O is done to achieve the same result, with more impact on the rest of the system. I haven't found any case or situation where avoiding the use of indices for sorting is faster.

Use of PDQ should speed up statements, not slow them down.

It would be an improvement if either:

  • where indices can be used for sorting, with PDQ set the database engine behaved the same as in the non-PDQ case.

  • even better: multiple indices could be read in parallel for the same table reducing the run time.

A compromise could be to use estimated costs to decide whether to scan the table or use indices for sort but certainly in my testing, using indices for sort was always faster and used less I/O by a considerable margin.

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