1. INTRODUCTION
Query processing is one of the most
critical activities that SQL Server performs in order to return data
from your T-SQL queries. Understanding how SQL Server processes
queries, including how they are optimized and executed, is essential to
understanding what SQL Server is doing and why it chooses a particular
way to do it.
2. QUERY PROCESSING
Query processing is performed by the
Relational Engine in SQL Server. It is the process of taking the T-SQL
statements you write and converting them into something that can make
requests to the Storage Engine and retrieve the results needed.
SQL Server takes four steps to process a query: parsing, algebrizing, optimizing, and execution. They are shown in Figure 1.
The first three steps are all performed by the
Relational Engine. The output of the third step is the optimized plan
that is scheduled, and during which calls are made to the Storage
Engine to retrieve the data that becomes the results of the query you
are executing.
Parsing
During the parsing stage SQL Server
performs basic checks on the source code (your T-SQL batch). This
parsing looks for invalid SQL syntax, such as incorrect use of reserved
words, column and table names, and so on.
If parsing completes without errors, it generates
a parse tree, which is passed onto the next stage of query processing,
binding. The parse tree is an internal representation of the query. If
parsing detects any errors, the process stops and the errors are
returned.
Algebrizing
The algebrization stage is also referred to as the binding stage. In early versions of SQL Server this stage was referred to as normalization.
During algebrizing, SQL Server performs several operations on the parse
tree and then generates a query tree that is passed on to the Query
Optimizer.
The steps performed during algebrizing follow this model:
- Step 1: Name resolution — Confirms
that all objects exist and are visible in the security context of the
user. This is where the table and column names are checked to ensure
that they exist and that the user has access to them.
- Step 2: Type derivation — Determines the final type for each node in the parse tree
- Step 3: Aggregate binding — Determines where to do any aggregations
- Step 4: Group binding — Binds any aggregations to the appropriate select list
Syntax errors are detected during this
stage. If a syntax error is encountered, the optimization process halts
and the error is returned to the user.
