In this short post I would like to point out a non-obvious issue that one of my customers recently hit. On the one hand, it’s a typical case where the query optimizer generates a different (suboptimal) execution plan even though nothing relevant (of course, at first sight only) was changed. On the other hand, in […]
V$SQL_CS_HISTOGRAMS: What Are the Buckets’ Thresholds?
The contents of the V$SQL_CS_HISTOGRAM view is used by the SQL engine to decide when a cursor is made bind aware, and therefore, when it should use adaptive cursor sharing. For each child cursor, the view shows three buckets. It is of general knowledge that the first one (BUCKET_ID equal 0) is associated with the […]
Partition-Wise Operations – New Features in 12c and 18c
Partition-wise operations are not something new. I do not remember when they were introduced, but at that time the release number was still a single digit. Anyway, the aim of this post is not to describe the basics, but only to describe what is new in that area in 12c and 18c. The new features […]
TKPROF’s Argument PDBTRACE
If you run TKPROF without arguments, you get a complete list of its arguments with a short description for each of them (here the output generated by version 18.1.0): $ tkprof Usage: tkprof tracefile outputfile [explain= ] [table= ] [print= ] [insert= ] [sys= ] [sort= ] table=schema.tablename Use ‘schema.tablename’ with ‘explain=’ option. explain=user/password Connect […]
CDB Views and Query Optimizer Cardinality Estimations
Today I faced a performance problem caused by a bad cardinality estimation involving a CDB view in a 12.1.0.2 multitenant environment. While solving the problem I did a number of observations that I try to summarize in this blog post. First of all, when checking the execution plan of a query already running for more […]
How Well a Query Optimizer Handles Subqueries?
At the beginning of December, at the UKOUG Tech17 conference in Birmingham (GB), I presented a comparison of the query optimizers of MySQL 8.0.3 and PostgreSQL 10.1. One of the things I talked about is their ability to handle subqueries. I summarized my findings with the following sentence: Simple sub-queries that are not correctly optimized […]
SQL_ADAPTIVE_PLAN_RESOLVED Is Broken
You can use the V$SQL.IS_RESOLVED_ADAPTIVE_PLAN column to know whether the execution plan associated to a child cursor is adaptive or not. Specifically, to know whether the query optimizer selected either an adaptive join method or an adaptive star transformation (notice that it is not set when the hybrid hash distribution is involved). The column takes […]
SPD State Does Not Change If Adaptive Statistics Are Disabled
The aim of this post is to point out an issue (bug?) that I recently discovered. But, before talking about it, I need to go through a rather long introduction about the state of SQL plan directive (SPD). As of version 12.1.0.2, an SPD has two state information. You can see both of them through […]
The APPROX_MEDIAN Function – A Test Case
The aim of this post is not to explain how the APPROX_MEDIAN function works (you find basic information in the documentation) but to show you the results of a test case I ran to assess how well it works. Here’s what I did… I started in the Oracle Database Public Cloud an instance of version […]
Activating and Deactivating Performance Feedback
Performance feedback is one of the adaptive query optimizer features introduced in Oracle Database 12c. The aim of this short post isn’t to explain how it works, but to simply warn you about a buggy behavior in its configuration. The parameters that control performance feedback in 12c are the following: OPTIMIZER_ADAPTIVE_FEATURES (12.1 only): if set […]