Jonathan Lewis

Jonathan Lewis is a well-known figure in the Oracle world with more than 30 years’ experience using the software. He has published three books about Oracle – the most recent being “Oracle Core” published by Apress Nov 2011 – and contributed to three others. He has posted more than 1,400 articles on his blog and has spent a lot of his time contributing to newsgroups, forums, and User Group magazines and events around the world. Jonathan has been self-employed for most of his time in the IT industry. For the last 25 years he has specialised in short-term assignments – typically of a design, review, or troubleshooting nature – often spending no more than two or three days on a client site to address problems. After visiting more than 50 countries (and more than a dozen US states) to talk about or troubleshoot Oracle systems, he has finally gone into near-retirement, though still writes articles and does a little consulting over the internet. Despite the differences in the software, he finds that the fundamental principles of solving performance issues don't really seem to change as you move from Oracle to SQL Server. His blog can be found at http://jonathanlewis.wordpress.com.
26 October 2022
26 October 2022

Oracle optimizer Or Expansion Transformations

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The previous installment of this series examined aggregate subquery removal and subquery coalescing, describing the latter as similar in some ways to an inverse for “Or Expansion” and “Join Factorization”. In this instalment, it’s time to take a closer look at Or Expansion and we’ll move on to Join Factorization in the next instalment.… Read more
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24 January 2017
24 January 2017

Snapshots

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While looking at the results from a query against the dynamic performance view v$active_session_history during a recent trouble-shooting assignment I noticed a result that was so unusual that I decided I needed to get some information from the v$ash_info view over an interval of about 3 minutes – and it didn’t matter which three minutes in particular, any three (consecutive)… Read more
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24 January 2017
24 January 2017

Index Costing Threat

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In the previous post I described a problem with the strategy that the optimizer uses for costing an index fast full scan, and the alternative strategy that Oracle had supplied at some point in the 10g timeline to allow a more rational cost to be calculated in special cases. In an earlier post still I had described the problem wit… Read more
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20 January 2017
20 January 2017

Problems with Indexes

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In my book “Cost Based Oracle – Fundamentals” (Apress 2005) I described a problem with the cost calculation for the index fast full scan: When you generate statistics for a table, one of the results is the number of blocks below the high water mark. When you generate the statistics for an index, you get the number of leaf blocks… Read more
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09 January 2017
09 January 2017

Investigating Indexes – 2

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In an earlier article on investigating indexes I described some methods for looking at the internal structure of an index to get an idea of its current state of health. Although this type of knowledge helps you to make informed decisions about which indexes might need special treatment (like being permanently dropped, perhaps) it doesn’t immediately answer questions like: “what’s… Read more
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03 October 2016
03 October 2016

Hacking Indexes

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Indexes are expensive to maintain so we want to get the greatest benefit we can from them and make sure that Oracle uses them whenever it’s appropriate. Sometimes, though, the optimizer seems to ignore indexes that we think would be very effective and when this happens it’s often because the statistics that Oracle has gathered do not reflect the quality… Read more
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28 September 2016
28 September 2016

Investigating Indexes

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Rebuilding indexes is an activity that you shouldn’t need to do often. There are always a few special cases – like when you’ve moved or recreated a table or if you’ve done a massive delete on a table – when it’s probably a reasonable idea but, in general, there are very few cases where there’s any great benefit to b… Read more
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05 September 2016
05 September 2016

Text Indexes

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The Oracle database has many features that are not spoken of frequently and a recent note by Connor McDonald reminded me that there are a couple of notes about optimising Text indexes that have been on my “must write” list for several years. If you’re not familiar with what text indexes can do for you, here’s a thumbnail sketch: creat… Read more
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05 September 2016
05 September 2016

Massive Deletes – Part 4

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In the previous installment of this series I produced some figures highlighting the main differences between doing a large delete by tablescan and doing a large delete by index range scan. Depending on the data patterns involved the correct choice of strategy could make a significant difference to the number of random I/Os, the volume of redo generated, and t… Read more
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05 April 2016
05 April 2016

Massive Deletes – Part 3

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In the previous installment of this series we looked at methods for seeing the pattern of the changes that would appear in a table and its indexes after a large delete. In this installment we move on to the workload implied by different patterns, then think about strategies or, indeed, the need for strategies for reducing that workload at t… Read more
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