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Global Hints
Occasionally I encounter a situation when I need to affect a part of the plan that corresponds to a view, e.g.: select * from ( select v.x, x.y from v ) q where q.x = 1 Such situations are resolved...
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Read consistency overhead
SQL performance can degrade for many reasons, some of most common are: - plan changes - data skewness - low caching efficiency - data growth - contention. All these factors are relatively well known. A...
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Querying trace files
SQL trace file provide the highest level of detail possible about SQL execution. The problem with that information is converting it to a convenient format for further analysis. One very good solution...
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Piggyback commits
Not every commit results in a redo write. This is because there are multiple optimizations (some controlled by the user e.g. with COMMIT_LOGGING parameter, some automatic) that aim at reducing the...
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CPU-starved LGWR
In my recent post I showed how log file sync (LFS) and log file parallel write (LFPW) look for normal systems. I think it would also be interesting to compare that to the situation when LGWR does not...
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Lies, damned lies and non production-like performance testing
Chasing cost efficiency, business often cuts back on money spent on UAT boxes used for performance testing. More often than not, this is a bad-decision, because the only thing worse than not having a...
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Log buffer space
Introduction Log buffer space is a simple, yet frequently misunderstood wait event. The main reason for that is probably its name. It sounds as if it points immediately to the answer: if space in the...
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Nested loop internals
Nested loop join appears like the simplest thing there could be — you go through one table, and as you go, per each row found you probe the second table to see if you find any matching rows. But thanks...
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Query tuning by waste minimization: a real-life example
Today I’d like to share another tuning example from a recent case at work, which in my opinion is good for illustrating typical steps involved in SQL optimization process. I was handed a poorly...
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Business rules, common sense and query performance
Very often, significant performance benefits can be obtained by using some very basic knowledge of the application, its data and business rules. Sometimes even less than that: even if you are not...
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