From SQL parse trees to RA expression trees

QueryStarsIn(title,year,starName)

MovieStar(name,address,gender,birthdate)

Query:

Give titles of movies that have at least one star born in 1960

SELECT titleFROM StarsIn, MovieStarWHERE starName = name AND birthdate LIKE '%1960%' ;

Parse Tree<Query>

<SFW>

SELECT <SelList> FROM <FromList> WHERE <Condition>

title StarsIn <RelName>

MovieStar

<Attribute> <RelName> , <FromList> AND

<Condition> <Condition>

<Attribute> = <Attribute> <Attribute> LIKE <Pattern>

starName name birthdate ‘%1960’

Rules (1)• If we have a <Query> with a <Condition> that has no

subqueries, then replace the entire construct –

the select-list, from-list, and condition –

by a relational-algebra expression (tree) consisting, from bottom to top, of:

1. The product of all the relations mentioned in the <FromList>, which is the argument of:

2. A selection C, where C is the <Condition> expression in the construct being replaced, which in turn is the argument of:

3. A projection L,where L is the list of attributes in the <SeIList>.

RA tree

Queries Involving Views (1)Operands in a query might be virtual views.

View: CREATE VIEW ParamountMovies AS SELECT title, year FROM Movies WHERE studioName = 'Paramount';

Views are represented by RA trees as well.

Query:SELECT titleFROM ParamountMoviesWHEREyear = 1979;

Leaf will be the view.

Queries Involving Views (2)• To get query over base tables, we substitute, for each view-

leaf, the root of the tree that defines that view.

Simplifying the query over base tables

Queries Involving Views (3)• Of course there might be more than one view used in a

query.

Subqueries in ConditionsExample

SELECT titleFROM StarsInWHERE starName IN (

SELECT nameFROM MovieStarWHERE birthdate LIKE '%1960'

);

intermediate form of operator, called

two-argument selection.

Removing Subqueries From ConditionsCase 1: Uncorrelated subqueries

Don’t produce more copies of tuples than

the original query does…

Condition that equates each component of a tuple t to the

corresponding attribute of relation S.

Removing Subqueries From ConditionsCase 1: Example

No here because name is a key for movie stars…

Removing Subqueries From ConditionCase 2: Correlated subqueries

Example

Find the movies where the average age of the stars was at most 40 when the movie was made.

SELECT DISTINCT mi.movieTitle, mi. movie YearFROM StarsIn m1WHERE m1. movie Year - 40 <= ( SELECT AVG(birthdate) FROM StarsIn m2, MovieStar s WHERE m2. starName = s. name AND m1.movieTitle = m2.movieTitle AND m1. movieYear = m2.movieYear);

Removing Subqueries From ConditionCase 2: Correlated subqueries

Intermediate form

Solution: Defer this selection…i.e. pull up the selection.

How?

Removing Subqueries From ConditionCase 2: Correlated subqueries

Solution

Removing Subqueries From ConditionCase 2: Correlated subqueries

Improving solution:The join between StarsIn m1 and the rest of the expression equates the title and year attributes from StarsIn m1 and StarsIn m2.

Oracle Plan

PLAN_TABLE• PLAN_TABLE holds execution plans generated by the EXPLAIN PLAN

statement.

• Important columns in PLAN_TABLE:

statement_id Unique identifier for each execution plan

operation The operation performed in one step of the execution plan, such as “table access”

options Additional information about the operation, such as “by index ROWID”

object_name Name of table, index, view, etc. accessed

id Step number in execution plan

parent_id Step number of parent step

EXPLAIN PLAN statementEXPLAIN PLAN FOR

SELECT /*+ RULE */ m1.movieTitle, m1.movieYear

FROM StarsIn m1

WHERE m1.movieYear-40 <=(

SELECT AVG(birthdate)

FROM StarsIn m2, MovieStar s

WHERE m2.starName=s.name AND

m1.movieTitle=m2.movieTitle AND

m1.movieYear = m2.movieYear

);

This inserts the query plan into the PLAN_TABLE.

We better delete any previous content of the table.

Extracting the plan treeCOL operation FORMAT a35

COL object_name FORMAT a30

SELECT id, parent_id,

LPAD (' ', LEVEL - 1) || operation || ' ' || options AS operation,

object_name

FROM PLAN_TABLE

START WITH id = 0

CONNECT BY PRIOR id = parent_id;

The plan ID PARENT OPERATION OBJECT_NAME

---- ------ ----------------------------------- ------------------------------

0 SELECT STATEMENT

1 0 FILTER

2 1 TABLE ACCESS FULL STARSIN

3 1 SORT AGGREGATE

4 3 MERGE JOIN

5 4 SORT JOIN

6 5 TABLE ACCESS FULL MOVIESTAR

7 4 SORT JOIN

8 7 TABLE ACCESS FULL STARSIN

Execution Plan Operations

FILTER

Read a set of row data and discard some rows based on various criteria. To determine the criteria, operations from a second input may need to be performed. Input: rows to be examined and, sometimes, an additional subordinate operation that must be performed for each row from the first input in order to evaluate criteria. Output: the rows from the first input that met the criteria.

Execution Plan OperationsTABLE ACCESS FULL

Perform a full table scan of the indicated table and retrieve all rows that meet criteria from the WHERE clause.

INDEX UNIQUE SCAN

Look up a complete key in a unique index.

INDEX RANGE SCAN

Look up a key in a non-unique index, or an incomplete key in a unique index.

TABLE ACCESS BY INDEX ROWID

Look up rows in a table by their ROWIDs.

Execution Plan Operations

NESTED LOOPS

Perform a join between two sets of row data using the nested loops algorithm.

HASH JOIN

Perform a join between two sets of row data using the hash join algorithm.

Execution Plan OperationsSORT JOIN

The input is sorted by the join column or columns in preparation for a join using the merge join algorithm.

MERGE JOIN

Perform a join between two sets of row data using the merge join algorithm.

SORT ORDER BY

Sort the input rows for the purpose of satisfying an ORDER BY clause.

SORT GROUP BY

The rows are sorted and grouped to satisfy a GROUP BY clause.