query compiler. the query compiler parses sql query into parse tree transforms parse tree into...
Post on 18-Dec-2015
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The Query Compiler• Parses SQL query into parse tree
• Transforms parse tree into expression tree (logical query plan)
• Transforms logical query plan into physical query plan
{P1,P2,…..}
{P1,C1>...}
parse
convert
apply laws
estimate result sizes
consider physical plans estimate costs
pick best
execute
Pi
answer
SQL query
parse tree
logical query plan
“improved” l.q.p
l.q.p. +sizesstatistics
Grammar for simple SQL<Query> ::= <SFW><Query> ::= (<Query>)
<SFW> ::= SELECT <SelList> FROM <FromList> WHERE <Cond>
<SelList> ::= <Attr>,<SelList><SelList> ::= <Attr>
<FromList> ::= <Relation>, <FromList><FromList> ::= <Relation>
<Cond> ::= <Cond> AND <Cond><Cond> ::= <Tuple> IN <Query><Cond> ::= <Attr> = <Attr><Cond> ::= <Attr> LIKE <Pattern>
<Tuple> ::= <Attr>
Atoms(constants), <syntactic categories>(variable),::= (can be expressed/defined as)
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>
<Attribute> <RelName> , <FromList> AND
title StarsIn <RelName>
<Condition> <Condition>
<Attribute> = <Attribute> <Attribute> LIKE <Pattern>
starName name birthdate ‘%1960’
MovieStar
Another query equivalent
SELECT titleFROM StarsInWHERE starName IN (
SELECT nameFROM MovieStarWHERE birthdate LIKE '%1960%'
);
The Preprocessor (expand query & semantic checking)
• Checks against schema definition:– Relation uses
– Attribute uses, resolve names ( A to R.A)
– Use of types (strings, integers, dates, etc)
and operators’ arguments type/arityThese preprocessing functions are called
semantic checking
• If all tests are passed, then the parse tree is said to be valid
Algebraic laws for transforming logical query plans• Commutative and associative laws:
)()( TSRTSR
RSSR
)()( TSRTSR
RSSR
Above laws are applicable for both sets and bags
Theta-join• Commutative:
• Not always associative: – On schema R(a,b), S(b,c), T(c,d) the first query can not be
transformed into the second: (Why?)
RSSR CC
TSR dabSbR )( ..
)(.. TSR dabSbR
Because, we can’t join S and T using the condition a<d since a is an attribute of neither S nor T.
Laws Involving Selection () ))(()( RR DCDC
)()()( RRR DSCDC
))(())(( RR CDDC
Splitting laws
Only if R is a set.
The union is
“set union”Order is flexible
Laws Involving Selection ()
)()()( SRSR CCC
SR
SR
SR
C
CC
C
)(
)()(
)(
What about intersection?
SRSR CC )()(
Algebraic Laws involving selection )(RDC
)( SRC
)( SRC
)( SR DC
For the binary operators, we push the selection only if all attributes in the condition C are in R.
))(( RDC
SR C
SRC )(
SR DC )(
Example:
• Consider relation schemas R(A,B) and S(B,C) and the expression below:
(A=1 OR A=3) AND B<C(R S)
1. Splitting AND A=1 OR A=3 (B < C(R S))
2. Push to S A=1 OR A=3 (R B < C(S))
3. Push to R A=1 OR A=3 (R) B < C(S)
Pushing selections• Usually selections are pushed down the expression tree.
• The following example shows that it is sometimes useful to pull selection up in the tree.
StarsIn(title,year,starName)Movie(title,year,length,studioName)
CREATE VIEW MoviesOf1996 AS SELECT * FROM MOVIE WHERE year=1996;
Query: Which stars worked for which studios in 1996?
SELECT starName,studioName FROM MoviesOf1996 NATURAL JOIN StarsIN;
Laws for (bag) Projection• A simple law: Project out attributes that are not needed
later. – i.e. keep only the input attr. and any join attribute.
)( SRL
)( SR CL
)( SRL
))()(( SR NML
))()(( SR NCML
))()(( SR NML
RCL RMC
Examples for pushing projection
Schema R(a,b,c), S(c,d,e)
)( SRxea ))(( , SR ecxea
)(, SRyedxba ))()(( ,,, SR cyedcxbayx
Example: Pushing Projection• Schema: StarsIn(title,year,starName) • Query: SELECT starName FROM StarsIn WHERE year = 1996;
Should we transform to ?Depends!Is StarsIn stored or computed?
starName
year=1996
StarsIn
starName
year=1996
StarsIn
starName,year
Reasons for not pushing the projection• If StarsIn is stored, then for the projection we have to scan
the relation.
• If the relation is pipelined from some previous computation, then yes, we better do the projection (on the fly).
• Also, if for example there is an index on year for StarsIn, such index is useless in the projected relation starName,year(StarsIn)
– While such an index is very useful for the selection on “year=1996”
Laws for duplicate elimination and grouping
Try to move in a position where it can be eliminated altogether
E.g. when is applied on • A stored relation with a declared primary key• A relation that is the result of a operation, since grouping creates a
relation with no duplicates.
)())(( RR LL absorbs
Improving logical query plans
• Push as far down as possible (sometimes pull them up first).
• Do splitting of complex conditions in in order to push even further.
• Push as far down as possible, introduce new early (but take care for exceptions)
• Combine with to produce -joins or equi-joins
• Choose an order for joins
Example of improvement
SELECT titleFROM StarsIn, MovieStarWHERE starName = name AND birthdate LIKE ‘%1960’;
title
starname=name AND birthdate LIKE ‘%1960’
StarsIn
MovieStar
title
StarsIn MovieStar
starName=name
birthdate LIKE ‘%1960’