The Query Compiler. 16. 1 Parsing and Preprocessing. Meghna Jain(205) The Query Compiler 16.1 Parsing and Preprocessing Meghna Jain(205) Dr. T. Y. Lin

Presentation Outline 16.1 Parsing and Preprocessing 16.1.1 Syntax Analysis and Parse Tree 16.1.2 A Grammar for Simple Subset of SQL 16.1.3 The Preprocessor 16.1.4 Processing Queries Involving Views

Query compilation is divided into three steps 1. Parsing: Parse SQL query into parser tree. 2. Logical query plan: Transforms parse tree into expression tree of relational algebra. 3.Physical query plan: Transforms logical query plan into physical query plan. . Operation performed . Order of operation . Algorithm used . The way in which stored data is obtained and passed from one operation to another.

Form a query to a logical query plan Parser Preprocessor Logical Query plan generator Query rewrite Preferred logical query plan Form a query to a logical query plan

Syntax Analysis and Parse Tree Parser takes the sql query and convert it to parse tree. Nodes of parse tree: 1. Atoms: known as Lexical elements such as key words, constants, parentheses, operators, and other schema elements. 2. Syntactic categories: Subparts that plays a similar role in a query as <Query> , <Condition>

Grammar for Simple Subset of SQL <Query> ::= <SFW> <Query> ::= (<Query>)‏ <SFW> ::= SELECT <SelList> FROM <FromList> WHERE <Condition> <SelList> ::= <Attribute>,<SelList> <SelList> ::= <Attribute> <FromList> ::= <Relation>, <FromList> <FromList> ::= <Relation> <Condition> ::= <Condition> AND <Condition> <Condition> ::= <Tuple> IN <Query> <Condition> ::= <Attribute> = <Attribute> <Condition> ::= <Attribute> LIKE <Pattern> <Tuple> ::= <Attribute> Atoms(constants), <syntactic categories>(variable), ::= (can be expressed/defined as)‏

Query and Parse T ree StarsIn(title,year,starName) MovieStar(name,address,gender,birthdate)‏ Query: Give titles of movies that have at least one star born in 1960 SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE birthdate LIKE '%1960%' );

Another query equivalent SELECT title FROM StarsIn, MovieStar WHERE starName = name AND birthdate LIKE '%1960%' ;

Parse Tree <Query> <SFW> SELECT <SelList> FROM <FromList> WHERE <Condition> <Attribute> <RelName> , <FromList> AND title StarsIn <RelName> MovieStar <Query> <Condition> <Condition> <Attribute> = <Attribute> <Attribute> LIKE <Pattern> starName name birthdate ‘%1960’

The Preprocessor Functions of Preprocessor . If a relation used in the query is virtual view then each use of this relation in the form-list must replace by parser tree that describe the view. . It is also responsible for semantic checking 1. Checks relation uses : Every relation mentioned in FROM- clause must be a relation or a view in current schema. 2. Check and resolve attribute uses: Every attribute mentioned in SELECT or WHERE clause must be an attribute of same relation in the current scope. 3. Check types: All attributes must be of a type appropriate to their uses.

StarsIn(title,year,starName) MovieStar(name,address,gender,birthdate)‏ Query: Give titles of movies that have at least one star born in 1960 SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE birthdate LIKE '%1960%' );

Preprocessing Queries Involving Views When an operand in a query is a virtual view, the preprocessor needs to replace the operand by a piece of parse tree that represents how the view is constructed from base table. Base Table: Movies( title, year, length, genre, studioname, producerC#)‏ View definition : CREATE VIEW ParamountMovies AS SELECT title, year FROM movies WHERE studioName = 'Paramount'; Example based on view: SELECT title FROM ParamountMovies WHERE year = 1979;

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