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Showing posts from November, 2017

But... we already have tools for explain plan

There are already ways to see execution plan This is very true. And for day to day use, they are more then sufficient.

But sometimes the tools don't show the functions/conditions correctly.
And some details are never present (e.g. in complex plans with multiple materialized subqueries, it's sometimes impossible to determine from which copy of a table/subquery a column comes).

(And out of scope of this series, you may want to see bind values, or to know which line in the plan you are on = which (copy of) table is oracle just reading.)

Examples in this blog are necessarily simple ones; and in such examples we see cases where the added value of such access to execution plan is not very high. But never forget that this can be extended to much more complex cases.
Existing tools EXPLAIN PLAN EXPLAIN PLAN is long time with us. The main problem is that it shows plan for a new parse - and this can be different than what a session is actually executing, as it depends on bind types and…

Reading data from PGA and SGA

Overview For our investigation of execution plan as it is stored in memory, we need in the first place to be able to read the memory.

We have the options of
x$ksmmem, reading SGA using SQL. Personally I don't like it, it's cumbersome and SGA read: obviously reading SGA only; it's fast and easy to doread process memory: can read PGA, process stack - and since the processes do map the SGA, too, you can read it as well. Unfortunately ptrace sends signals to the processes and the process is paused when reading it, but so far all my reads were short and fast and the processes did not notice. Some OS configurations can prevent you from using ptrace (e.g. docker by default), google for CAP_SYS_PTRACE.gdb: using your favorite debugger, you can read memory as well. Useful when investigating. Direct SGA read I always considered direct SGA read of some dark magic, but the fundamentals are actually very easy. It still looks like sorcery when actually reading the Oracle in…

Reading execution plan from SGA and PGA - teaser

Some of you have seen my presentation about hidden parts of Oracle execution plans and how to access the plan in the memory directly and parse it. I presented it at OakTable World 2017  and it will be also presented in Wellington and Acukland this November.  You can download the presentation at

I realize that many people did not have the chance to attend... and that the slides need quite a lot of explaining, it's hard to understand this internals without more explanation.

Thus you can look forward to seeing a handful of blog posts, inspired by this presentation, and explain some of the aspects of this whole topic.

We will start with some basic tools; accessing SGA and PGA, which you may find useful for many other tasks, too. Then we will look at some details of the execution plan. This is not - and neither the presentation is - an exhaustive guide to the execution plan internals. It would be a multi-year project to t…