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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 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 lengths, NLS settings, and SQL baselines, many optimizer settings... and every version of Oracle introduces more dynamics and adaptability, making it harder and harder to get exactly the same plan.
On the other hand, EXPLAIN PLAN runs "offline" and thus has time to do more parsing of the plan than other approaches, and thus can parse some cases that other ways cannot. E.g. this output of explain plan won't be shown with dbms_xplan:
But even explain plan fails sometimes:
filter(INTERNAL_FUNCTION("TIME_ID")=TIMESTAMP' 2000-01-01 00:00:00.000000000')
And in other cases (like the multiple subqueries/tables), it simply cannot show the full info, as the plan table does not allow that level of detail.


With dbms_xplan, it is easier to get the right plan, as we can specify an existing plan id, or existing cursor.  But it does not parse some of the more complicated cases - for example the first filter from above is shown as:


The output is similar to dbms_xplan. What is nice that this is a v$ view... so it should be backed up by a x$ view, and that should have an address in memory, something we should be able to read from the SGA. Let's have a look.

An example SQL

Our example is based on the sample SH schema, with an added table FOOBAR (id number, key varchar2(30)):

SELECT prod_id, key FROM products CROSS JOIN foobar WHERE prod_id in (143,144,id) and id in (1,2,3);

So let's have a look at the v$sql_plan. This is backed by x$kqlfxpl:

select addr, kqlfxpl_oper, kqlfxpl_oopt from x$kqlfxpl where kqlfxpl_sqlid='b4hdxqwy614fa'
---------------- -------------------- -------------------
00007F9F384F2990 TABLE ACCESS         FULL
00007F9F384F2698 INDEX                FULL SCAN
The important detail is the ADDR. As we've seen in the previous post, SGA base is 0x60_000_000, but e.g the first ADDR is 
0x7F9_F38_4F2_E68, somewhere very, very far. It's also close to 2^48, the size of current CPU address space (64-bit CPUs use 48-bit address space, to save on the address line cost; it's still more than enough). Process stack tends to grow down from the top of process address space. This indicates that this is process' private stack memory.

The truth is that v$sql_plan does not read the plan directly - instead it is using a helper function to first populate a temporary array, and then to show that array. We thus need to go deeper to get the raw, unparsed execution plan.

Next time we will look where the function gets the data from.


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