Optimizer catalogue

This is a list of all articles I’ve written that talk about the mechanisms the optimizer can choose. Each entry has a date stamp and a short note of the contents. The articles are (may eventually be) grouped in a couple of different categories, but otherwise are generally listed most-recent first.

I am currently working backwards through my entire blog adding articles to catalogues as I go, so any individual catalogue may grow very slowly. I also have a separate “Transformations” catalogue available from the main catalogue.

• Removing Subqueries (Oct 2023): notes on how the optimizer can use analytic (window) functions to remove some subqueries from your SQL – and why it may not be happening right now.
• Join view delete (May 2022): ANSI expansion problems with deletes from join views
• Adaptive Plans (April 2022): what they can cost and how block then selectively
• Generated predicates (March 2022): virtual columns from “function-based” indexes may allow the optimizer to generate predicates that you weren’t expecting to see
• Hash Aggregation 1 (Jan 2022): you may need to hint hash aggregation for some queries that group by then order by the same columns
• Hash Aggregation 2 (Jan 2022): Workarea operations listed in execution plans may have difference names in the v$sql_workarea.
• use_nl() redux (Oct 2021): where in its processing does Oracle apply join method hints like use_nl()
• A little tweak in 19c (July 2021): the optimizer adding an “is not nulll” predicate that changes the cost of an execution path.
• Join Elimination bug (Mar 2021): inconsistent behaviour when combined with deferrable constraints.
• use_nl_with_index (Mar 2021): a description of what it can do, and one (current) omission
• Adaptive Plan bug (Feb 2021): An adaptive plan (on partitioned tables with missing statistics) producing wrong results even in 21.3
• Optimizer arithmetic (Jan 2021): A little case study of the optimizer’s cardinality calculations.
• Between (Jan 2021): a small optimisation detail that appears for “between” clauses.
• Index descending First N (Dec 2020): until 19c an first rows in descending order had to be hinted with index_desc()
• Count Rows (Feb 2020): A note about counting rows in a table, with links going back back 20 years.
  • Count transformation (Oct 2008): Oracle treats many things as the same as count(*)
  • Counting union all (Mar 2012): A case where Oracle doesn’t do the optimum transformation
  • Count not transformed (Oct 2012): A crack in the “count(constant)” -> “count(*)” transformation
  • Count transformation redux (Jan 2015): Looking at the “unparsed” query for confirmation.
  • Counting rowids by accident (Jan 2015): Cursor_Sharing and other exceptions to “count conversion”.
  • A counting performance anomaly (Jan 2015): Not count(*) but an internal transformation could do less work and invites a manual transformation.
• “In-table” predicates (April 2019): how does Oracle handle tabX.col1 = tabX.col2 when there are histograms on the columns
• nvl() vs. coalesce() (Feb 2018): a case for using nvl() – because coalesce gives bad cardinality estimates.
  • coalesce() vs.nvl() (Jan 2014): a case for using coalesce() – because it “short-circuits” and nvl() doesn’t – but with a warning.
• Character Selectivity (Apr 2017): How does the optimizer calculate the cardinality of column like ‘xxx%’
• Index to Simpletalk series on CBO (2015): link to an introductory series of 6 articles on the basics of how the optimizer works that I wrote for Simpletalk, with a link to a Chinese translation
• Quantum Data (Nov 2014): an inconsistency in cardinality estimates for “out of range” data – still the same in the latest versions
• Index FFS Deletes (June 2014): From 12.2 Oracle can use an index fast full scan to drive a delete (or, in limited cases) an update. Still not able to drive a select.
• index_combine() limitation (June 2011): A (not unsurprising) limitation first reported in 11g, still present in 23c.
• Optimizer Environment (May 2007): seeing details of the optimizer parameter settings in v$sql_optimzer_env etc. (Updated to 23c).

External Links

• Index to Simpletalk series on CBO (2015): List of 6 articles in a series on the basics of how the optimizer works, written for Simpletalk, with a link to a Chinese translation

Other Authors

• Index to Stefan Koehler’s articles on the CBO (2015): a list of articles published on the SAP knowledgebase between 2012 and 2015