Slideshow:
Steps to process a SQL query
Anatomy of the Query Compiler
The Query Parser
The Query Re-writing module
The Query Re-writing module
The Physical Query Plan Generation module
Example:
Order of discussion
- Steps needed to
process a
query (SQL command):
Note:
- We will discuss the
Query Execution
first
- The Query Compiler/Interpreter will be discussed next (after Query Execution )
- We will discuss the
Query Execution
first
- The Query Compiler
(which will be discussed later)
consists of 3 major steps:
Explanation:
- Parser:
- Parses the SQL command and constructs a parse tree that represents the syntax elements in the SQL command
Example:
- Query Re-writing:
- First,
converts a
parse tree into
an un-optimized logical query plan
- A logical query plan consists of Relational Algebra operators
Example:
- Then,
converts the
un-optimized logical query plan
into an
optimized logical query plan
Example:
The optimized logical query plan is a.k.a. the logical query plan
- First,
converts a
parse tree into
an un-optimized logical query plan
- Physical Query Plan Generation:
- Select the
best algorithm to
execute the
logical query plan
- Usually, there are
multiple algorithms
available
to implement
one relation algebra
operation
- We select
the best algorithm
depending on:
- Availabilty of
indexes
- How much
main memory
is available for
query processing
(Fast algorithms require more memory)
- Availabilty of
indexes
Example: choosing an algorithm for relational algebra operators
- Usually, there are
multiple algorithms
available
to implement
one relation algebra
operation
- Select the
best algorithm to
execute the
logical query plan
- Parser:
- Query Compilation will
be discussed later
- In order to
provide you some
context
(to understand Query Execution),
we will now
discuss:
- an abbreviated presentation of Query Compilation
- More specifically:
- How do we
decide which
logical query plan is
better ???
Answer: lowest cost ...
- How do we
decide which
logical query plan is
better ???