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Showing posts from December, 2017

Filter and access predicates

More than just column projections When we look around for further pointers in the tree nodes, we find more pieces resembling the column projections we have seen so far. With some experimenting, we will find out that these are access predicates and filters.

First of all, the location of these pointers is not always the same, it seems that the value at 0x34 is some kind of flag, indicating whether filters and/or access predicates are present, and where. Or whether there is just one, or more of them.  It probably also indicates what other info is present, but I have no idea what info that would be or what each value means.
Resembling, but different The data we see as predicates are not columns; after all, a predicate is a condition, not a single column. But the structure is similar to what we have seen with columns, and if we follow pointers further, we eventually build a tree, and some of the leaves are indeed just column projections.
After some contemplation, we realize it's all t…

Column projections

The lower half of the plan In the previous part, we've seen the data behind the execution plane table, i.e. all the operations, their cost, and the tree structure of the plan. Although interesting to investigate, it's all already available in the dbms_xplan output. Although possibly you can find cases when it does, perhaps with some more complex parallel queries, partitioning and so on.

The more complex part is the "lower half" of the plan, i.e. the column projections, filters and access predicates. These can contain arbitrary operations/functions and it's also here where we can follow where a value comes from, starting from a single table, going through joins, aggregations, set functions and so on, up to the level where is it used (be it top-level column projection = in the select output, or as a filter or access predicate).

In this blog post, let's look at the column projections. In other words, what columns are shown in the resulting select, as well as ev…

Execution plan rows

The plan As mentioned in previous post, 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);

In all reverse engineering, it's good to start with something simple and to know what we to look for. Thus we want to know what the execution plan should look like; and the more unique numbers/ids we can find, the better. It's much easier to look for a number like 0x12fa1893 than for 0x0 or 0x1.

The execution plan, obtained using:
I have added the CPU cost from the full detail of the execution plan in  v$sql_plan / x$kqlfxpl.

Looking at the numbers, we also have rows (1 and 2 ... not very unique), bytes (34 / 30 / 8 - not bad) and what is not shown here, we also know object ids of the index and the table: 94765 and 92749 (nice).
We did not use any temp space and …