Oracle -| How to read an Explain Plan in Oracle
How to Interpret an Explain Plan
What is an explain plan? A statement's execution plan is the sequence of operations Oracle performs to run the statement.
An ordering of the tables referenced by the statement
An access method for each table mentioned in the statement
A join method for tables affected by join operations in the statement
Data operations like filter, sort, or aggregation
The DBMS_XPLAN package is used to format the output of an explain plan
ID=0 has no operation above it, so it has no parent but it has 1 child.
ID=0 is the parent of ID=1 and is dependent upon it for rows.
You can tell it is the parent because the child is indented.
So ID=1 must be executed prior to ID=0
Moving on to ID=1
As before, ID=1 is the child of ID=0.
From the indentation,
ID=2 and ID=4 are indented at the same level beneath ID=1.
Thus ID=1 is the parent of ID=2 and ID=4 and is dependent upon them for rows.
So ID=2 and ID=4 must be executed prior to ID=1
ID=2 is the first child of ID=1.
From the indentation,
ID=2 is the parent of ID=3 and is dependent upon it for rows.
So ID=3 must be executed prior to ID=2. Moving on to ID=3:
ID=3 is the (only) child of ID=2.
ID=3 has no child operations.
This means that ID=3 is the first step that is executed by the query.
Rows are provided to ID=2 from this step
ID=4 is the second child of ID=1.
ID=4 is the parent of ID=5 and is dependent upon it for rows. ID=5 must be executed prior to ID=4. This means that ID=5 is the third step that is executed followed by ID=4.