Download Query Processing Principles & Tuning Queries

L05: Query Processing & Tuning Queries
Query Processing -- Principles
 Tuning Queries

H.Lu/HKUST
Query Processing & Optimization





Any high-level query (SQL) on a database must be processed,
optimized and executed by the DBMS
The high-level query is scanned, and parsed to check for
syntactic correctness
An internal representation of a query is created, which is
either a query tree or a query graph
The DBMS then devises an execution strategy for retrieving
the result of the query. (An execution strategy is a plan for
executing the query by accessing the data, and storing the
intermediate results)
The process of choosing one out of the many execution
strategies is known as query optimization
H.Lu/HKUST
L04: Physical Database Design (2) -- 2
Query Processor & Query Optimizer
A query processor is a module in the DBMS that
performs the tasks to process, to optimize, and to
generate execution strategy for a high-level query
 Queries expressed in SQL can have multiple
equivalent relational algebra query expressions. The
query optimizer must select the optimal one

H.Lu/HKUST
L04: Physical Database Design (2) -- 3
Database Query Processing
Quer y
Par ser
I nt ernal
Represent at i on
Quer
Opt i mi zor
Quer y
Resul t s
Runt i me
Suppor t
Dat abase
H.Lu/HKUST
St or age
Query
Execut i on Pl an
Execut abl e
Code
Code
Gener at i on
St at i st i cs
Dat abase
L04: Physical Database Design (2) -- 4
Relational Operations

We will consider how to implement:
 Selection : Selects a subset of rows from relation.
 Projection : Deletes unwanted columns from
relation.
 Join : Allows us to combine two relations.

Since each operation returns a relation, opreations
can be composed!
H.Lu/HKUST
L04: Physical Database Design (2) -- 5
Simple Selections
SELECT *
FROM Reserves R
WHERE R.rname < ‘C%’
Of the form  R . a ttr o p v a lu e ( R )
 Size of result approximated as size of R * reduction
factor; we will consider how to estimate reduction
factors later.
 With no index, unsorted: Must essentially scan the
whole relation; cost is |R| (# pages in R).
 With an index on selection attribute: Use index to
find qualifying data entries, then retrieve
corresponding data records. (Hash index useful only
for equality selections.)

H.Lu/HKUST
L04: Physical Database Design (2) -- 6
Using an Index for Selections


Cost depends on #qualifying tuples, and clustering.
 Cost of finding qualifying data entries (typically small)
plus cost of retrieving records (could be large w/o
clustering).
 In example, assuming uniform distribution of names, about
10% of tuples qualify (100 pages, 10000 tuples). With a
clustered index, cost is little more than 100 I/Os; if
unclustered, upto 10000 I/Os!
Important refinement for unclustered indexes:
 1. Find qualifying data entries.
 2. Sort the rid’s of the data records to be retrieved.
 3. Fetch rids in order. This ensures that each data page is
looked at just once (though # of such pages likely to be
higher than with clustering).
H.Lu/HKUST
L04: Physical Database Design (2) -- 7
Selection - Summary
R.attr op value (R)
 Size of result = R * selectivity
 Processing methods
 Table scan
 Index scan

SELECT *
FROM Reserves R
WHERE R.rname < ‘C%’
• B+-tree, Hash index
• Clustered vs. non-clustered
– Clustered index: Good
– Non-clustered index:
• Good for low selectivity
• Worse than scan for high selectivity
H.Lu/HKUST
L04: Physical Database Design (2) -- 8
General Selection Conditions
(day<8/9/94 AND rname=‘Paul’) OR bid=5 OR sid=3



Such selection conditions are first converted to conjunctive
normal form (CNF):
(day<8/9/94 OR bid=5 OR sid=3 ) AND (rname=‘Paul’ OR
bid=5 OR sid=3)
We only discuss the case with no ORs (a conjunction of terms
of the form attr op value).
An index matches (a conjunction of) terms that involve only
attributes in a prefix of the search key.
 Index on <a, b, c> matches a=5 AND b= 3, but not b=3.
H.Lu/HKUST
L04: Physical Database Design (2) -- 9
Two Approaches to General Selections

First approach: Find the most selective access path, retrieve
tuples using it, and apply any remaining terms that don’t
match the index:
 Most selective access path: An index or file scan that we
estimate will require the fewest page I/Os.
 Terms that match this index reduce the number of tuples
retrieved; other terms are used to discard some retrieved
tuples, but do not affect number of tuples/pages fetched.
 Consider day<8/9/94 AND bid=5 AND sid=3. A B+ tree
index on day can be used; then, bid=5 and sid=3 must be
checked for each retrieved tuple. Similarly, a hash index
on <bid, sid> could be used; day < 8/9/94 must then be
checked.
H.Lu/HKUST
L04: Physical Database Design (2) -- 10
Intersection of Rids

Second approach (if we have 2 or more matching indexes that
use Alternatives (2) or (3) for data entries):
 Get sets of rids of data records using each matching index.
 Then intersect these sets of rids (we’ll discuss intersection
soon!)
 Retrieve the records and apply any remaining terms.
 Consider day<8/9/94 AND bid=5 AND sid=3. If we have a
B+ tree index on day and an index on sid, both using
Alternative (2), we can retrieve rids of records satisfying
day<8/9/94 using the first, rids of recs satisfying sid=3
using the second, intersect, retrieve records and check
bid=5.
H.Lu/HKUST
L04: Physical Database Design (2) -- 11
The Projection Operation

SELECT DISTINCT
FROM
R.sid, R.bid
Reserves R
An approach based on sorting:
 Modify Pass 0 of external sort to eliminate unwanted
fields. Thus, runs of about 2B pages are produced, but
tuples in runs are smaller than input tuples. (Size ratio
depends on # and size of fields that are dropped.)
 Modify merging passes to eliminate duplicates. Thus,
number of result tuples smaller than input. (Difference
depends on # of duplicates.)
 Cost: In Pass 0, read original relation (size M), write out
same number of smaller tuples. In merging passes, fewer
tuples written out in each pass. Using Reserves example,
1000 input pages reduced to 250 in Pass 0 if size ratio is
0.25
H.Lu/HKUST
L04: Physical Database Design (2) -- 12
Projection Based on Hashing



Partitioning phase: Read R using one input buffer. For each
tuple, discard unwanted fields, apply hash function h1 to
choose one of B-1 output buffers.
 Result is B-1 partitions (of tuples with no unwanted
fields). 2 tuples from different partitions guaranteed to be
distinct.
Duplicate elimination phase: For each partition, read it and
build an in-memory hash table, using hash fn h2 (<> h1) on
all fields, while discarding duplicates.
 If partition does not fit in memory, can apply hash-based
projection algorithm recursively to this partition.
Cost: For partitioning, read R, write out each tuple, but with
fewer fields. This is read in next phase.
H.Lu/HKUST
L04: Physical Database Design (2) -- 13
Discussion of Projection



Sort-based approach is the standard; better handling of skew
and result is sorted.
If an index on the relation contains all wanted attributes in its
search key, can do index-only scan.
 Apply projection techniques to data entries (much
smaller!)
If an ordered (i.e., tree) index contains all wanted attributes as
prefix of search key, can do even better:
 Retrieve data entries in order (index-only scan), discard
unwanted fields, compare adjacent tuples to check for
duplicates.
H.Lu/HKUST
L04: Physical Database Design (2) -- 14
Equality Joins With One Join Column
SELECT *
FROM Reserves R1, Sailors S1
WHERE R1.sid=S1.sid
In algebra: R  S. Common! Must be carefully
optimized.
 R
S is large; so, R S followed by a selection is
inefficient.
 Cost metric: # of I/Os. We will ignore output costs.


H.Lu/HKUST

L04: Physical Database Design (2) -- 15
Notations
|R| = number of pages in outer table R
 ||R|| = number of tuples in outer table R
 |S| = number of pages in inner table S
 ||S|| = number of tuples in inner table S
 M = number of main memory pages allocated

H.Lu/HKUST
L04: Physical Database Design (2) -- 16
Example of Join
sid
22
28
31
44
58
sname rating age
dustin
7
45.0
yuppy
9
35.0
lubber
8
55.5
guppy
5
35.0
rusty
10 35.0
sid sname
31 lubber
58 rusty
H.Lu/HKUST
rating
8
10
sid bid
day
31 101 10/11/96
58 103 11/12/96
age
55.5
35.0
rname
lubber
dustin
SELECT *
FROM Sailors R, Reserve S
WHERE R.sid=S.sid
bid
101
103
day
rname
10/11/96 lubber
11/12/96 dustin
L04: Physical Database Design (2) -- 17
Schema for Examples
Sailors (sid: integer, sname: string, rating: integer, age: real)
Reserves (sid: integer, bid: integer, day: dates, rname: string)



Similar to old schema; rname added for variations.
Reserves:
 Each tuple is 40 bytes long, 100 tuples per page, 1000
pages.
Sailors:
 Each tuple is 50 bytes long, 80 tuples per page, 500 pages.
H.Lu/HKUST
L04: Physical Database Design (2) -- 18
Simple Nested Loops Join
foreach tuple r in R do
foreach tuple s in S do
if ri == sj then add <r, s> to result


For each tuple in the outer relation R, we scan the entire inner
relation S.
 Cost: |R| + ||R|| * |S| = 1000 + 100*1000*500 I/Os.
Page-oriented Nested Loops join: For each page of R, get
each page of S, and write out matching pairs of tuples <r, s>,
where r is in R-page and S is in S-page.
 Cost: |R| + |R|*|S| = 1000 + 1000*500
 If smaller relation (S) is outer, cost = 500 + 500*1000
H.Lu/HKUST
L04: Physical Database Design (2) -- 19
Block Nested Loops Join

Use one page as an
input buffer for
scanning the inner S,
one page as the output
buffer, and use all
remaining pages to
hold ``block’’ of outer
R.
 For each matching
tuple r in R-block, s
in S-page, add <r,
s> to result.
 Then read next Rblock, scan S, etc.
H.Lu/HKUST
Read in
block by
block
Hash Table for R
(M-2) pages
h( )
Input
Buffer
Output
Buffer
Read in
page by
page
S
R
R
S
L04: Physical Database Design (2) -- 20
Block Nested Loop Join
Scan inner table S per block of (M – 2) pages of R
tuples
 Each scan costs |S| pages
 |R| / (M – 2) blocks of R tuples
 |R| pages for outer table R
 Total cost = |R| + |R| / (M – 2) * |S| pages
 R should be the smaller table

H.Lu/HKUST
L04: Physical Database Design (2) -- 21
Examples of Block Nested Loops




Cost: Scan of outer + #outer blocks * scan of inner
 #outer blocks =no of pages of outer / blocksize
With Reserves (R) as outer, and 100 pages of R:
 Cost of scanning R is 1000 I/Os; a total of 10 blocks.
 Per block of R, we scan Sailors (S); 10*500 I/Os.
 If space for just 90 pages of R, we would scan S 12 times.
With 100-page block of Sailors as outer:
 Cost of scanning S is 500 I/Os; a total of 5 blocks.
 Per block of S, we scan Reserves; 5*1000 I/Os.
With sequential reads considered, analysis changes: may be
best to divide buffers evenly between R and S.
H.Lu/HKUST
L04: Physical Database Design (2) -- 22
Index Nested Loops Join


Use one page as an
input buffer for
scanning the outer R,
one page as the output
buffer.
Use all remaining
pages to hold index of
inner S.
 For each tuple r in
R- page, probe
index of S to find
matches, s, add <r,
s> to result.
 Then read next Rpage, repeat.
H.Lu/HKUST
Read in
page by
page
Input
Buffer
Index
for S
S
R
Output
Buffer
R
S
L04: Physical Database Design (2) -- 23
Index Nested Loop Join
Probe S index for matching S tuples per R tuple
 Probe hash index: 1.2 I/Os
 Probe B+ tree: 2-4 I/Os, plus retrieve matching S
tuples: 1 I/O
 For ||R|| tuples
 |R| pages for outer table R
 Total cost = |R| + ||R|| * index retrieval
 Better than Block NL join only for small number of
R tuples

H.Lu/HKUST
L04: Physical Database Design (2) -- 24
Examples of Index Nested Loops


Hash-index (Alt. 2) on sid of Sailors (as inner):
 Scan Reserves: 1000 page I/Os, 100*1000 tuples.
 For each Reserves tuple: 1.2 I/Os to get data entry in
index, plus 1 I/O to get (the exactly one) matching Sailors
tuple. Total: 220,000 I/Os.
Hash-index (Alt. 2) on sid of Reserves (as inner):
 Scan Sailors: 500 page I/Os, 80*500 tuples.
 For each Sailors tuple: 1.2 I/Os to find index page with
data entries, plus cost of retrieving matching Reserves
tuples. Assuming uniform distribution, 2.5 reservations
per sailor (100,000 / 40,000). Cost of retrieving them is 1
or 2.5 I/Os depending on whether the index is clustered.
H.Lu/HKUST
L04: Physical Database Design (2) -- 25
Sort-Merge Join



Sort R and S on the join column
Scan sorted R and S to do a ``merge’’ (on join col.), and
output result tuples.
 Advance scan of R until current R-tuple >= current S
tuple, then advance scan of S until current S-tuple >=
current R tuple; do this until current R tuple = current S
tuple.
 At this point, all R tuples with same value in Ri (current R
group) and all S tuples with same value in Sj (current S
group) match; output <r, s> for all pairs of such tuples.
 Then resume scanning R and S.
R is scanned once; each S group is scanned once per matching
R tuple. (Multiple scans of an S group are likely to find
needed pages in buffer.)
H.Lu/HKUST
L04: Physical Database Design (2) -- 26
External Sort ( K-Way Merge Sort)
3
4
6
2
9
4
8
7
5
6
3
1
2
Input Data
Sorting
3
4
6
2
4
9
5
7
8
1
3
6
2
Sorted runs (L=3)
First merge
2
3
4
4
5
6
7
8
9
1
2
3
6
Sorted runs (L=9)
Second merge
1
H.Lu/HKUST
2
2
3
3
4
4
5
6
6
7
8
9
Sorted data
L04: Physical Database Design (2) -- 27
Sort Merge Join
External-sort R: 2 |R| * (logM-1 |R|/M + 1)
 Split R into |R|/M sorted runs each of size M: 2 |R|
 Merge up to (M – 1) runs repeatedly
 logM-1 |R|/M passes, each costing 2 |R|
 External-sort S: 2 |S| * (logM-1 |S|/M + 1)
 Merge matching tuples from sorted R and S: |R| + |S|
 Total cost = 2 |R| * (logM-1 |R|/M + 1) + 2 |S| *
(logM-1 |S|/M + 1) + |R| + |S|
 If |R| < M*(M-1), cost = 5 * (|R| + |S|)

H.Lu/HKUST
L04: Physical Database Design (2) -- 28
Example of Sort-Merge Join
sid
22
28
31
44
58
sname rating age
dustin
7
45.0
yuppy
9
35.0
lubber
8
55.5
guppy
5
35.0
rusty
10 35.0
sid
28
28
31
31
31
58
bid
103
103
101
102
101
103
day
12/4/96
11/3/96
10/10/96
10/12/96
10/11/96
11/12/96
rname
guppy
yuppy
dustin
lubber
lubber
dustin
Cost of merge is M+N, could be M*N (very unlikely!)
 With 35, 100 or 300 buffer pages, both Reserves and
Sailors can be sorted in 2 passes; total join cost: 7500.

(BNL cost: 2500 to 15000 I/Os)
H.Lu/HKUST
L04: Physical Database Design (2) -- 29
Hash Join – The Principle
Ri
R
S
n
Ri
Si
i 1
If
Ri
Sj  
S
Si
for i  j
R
H.Lu/HKUST
L04: Physical Database Design (2) -- 30
Original
Relation
OUTPUT
1
Hash-Join


Partition Phase:
Partition both
relations using hash
fn h1: R tuples in
partition i will only
match S tuples in
partition i.
Joining Phase:
Read in a partition of
R, hash it using h2
(<> h1!). Scan
matching partition of
S, search for matches.
H.Lu/HKUST
Partitions
1
2
INPUT
2
hash
function
...
h1
M-1
M-1
Disk
M main memory buffers
Partitions
of R & S
Disk
Join Result
hash
fn
Hash table for partition
Ri (k < M -1 pages)
h2
h2
Input buffer
for Si
Disk
Output
buffer
M main memory buffers
Disk
L04: Physical Database Design (2) -- 31
Hash Join Algorithm




Partition phase: 2 (|R| + |S|)
 Partition table R using hash function h1: 2 |R|
 Partition table S using hash function h1: 2 |S|
 R tuples in partition i will match only S tuples in partition i
 R  (M – 1) partitions, each of size |R| / (M – 1)
Join phase: |R| + |S|
 Read in a partition of R (|R| / (M – 1) < M -1)
 Hash it using function h2 (<> h1!)
 Scan corresponding S partition, search for matches
Total cost = 3 (|R| + |S|) pages
Condition: M > √f|R|, f ≈ 1.2 to account for hash table
H.Lu/HKUST
L04: Physical Database Design (2) -- 32
General Join Conditions


Equalities over several attributes (e.g., R.sid=S.sid AND
R.rname=S.sname):
 For Index NL, build index on <sid, sname> (if S is inner);
or use existing indexes on sid or sname.
 For Sort-Merge and Hash Join, sort/partition on
combination of the two join columns.
Inequality conditions (e.g., R.rname < S.sname):
 For Index NL, need (clustered!) B+ tree index.
• Range probes on inner; # matches likely to be much higher than
for equality joins.
 Hash Join, Sort Merge Join not applicable.
 Block NL quite likely to be the best join method here.
H.Lu/HKUST
L04: Physical Database Design (2) -- 33
Impact of Buffering


If several operations are executing concurrently, estimating
the number of available buffer pages is guesswork.
Repeated access patterns interact with buffer replacement
policy.
 e.g., Inner relation is scanned repeatedly in Simple Nested
Loop Join. With enough buffer pages to hold inner,
replacement policy does not matter. Otherwise, MRU is
best, LRU is worst (sequential flooding).
 Does replacement policy matter for Block Nested Loops?
 What about Index Nested Loops? Sort-Merge Join?
H.Lu/HKUST
L04: Physical Database Design (2) -- 34
Summary of Join Operator
Simple nested loop: |R| + ||R|| * |S|
 Block nested loop: |R| + |R| / (M – 2) * |S|
 Index nested loop: |R| + ||R|| * index retrieval
 Sort-merge: 2 |R| * (logM-1 |R|/M + 1) + 2 |S| *
(logM-1 |S|/M + 1) + |R| + |S|
 Hash Join: 3 * (|R| + |S|)
 Condition: M > √f|R|

H.Lu/HKUST
L04: Physical Database Design (2) -- 35
Overview of Query Processing
Statistics
Parser
Parsed Query
High Level Query
H.Lu/HKUST
Cost Model
Query
Optimizer
QEP
Database
Query
Evaluator
Query Result
L04: Physical Database Design (2) -- 36
Query Optimization
SELECT S.sname
FROM Reserves R, Sailors S
WHERE R.sid=S.sid AND
R.bid=100 AND S.rating>5
sname
bid=100
rating > 5
sid=sid




Given: An SQL query joining n tables
Dream: Map to most efficient plan
Reserves
Reality: Avoid rotten plans
State of the art:
 Most optimizers follow System R’s technique
 Works fine up to about 10 joins
H.Lu/HKUST
Sailors
L04: Physical Database Design (2) -- 37
Complexity of Query Optimization

Many degrees of freedom
 Selection: scan versus
(clustered, nonclustered) index
 Join: block nested loop,
sort-merge, hash
 Relative order of the
operators
 Exponential search
space!
A
H.Lu/HKUST

Heuristics
 Push the selections
down
 Push the projections
down
 Delay Cartesian
products
 System R: Only leftD deep trees
C
B
L04: Physical Database Design (2) -- 38
Equivalences in Relational Algebra

Selection:  c1... cn  R 
 c1  . . .  cn  R
- cascade
 c1  c 2  R    c 2  c1  R - commutative



Projection:  a1  R   a1 . . .  an  R 

Join:
R  (S T)
(R S)
H.Lu/HKUST
 (RS)  T
 (S R)
- cascade
- associative
- commutative
L04: Physical Database Design (2) -- 39
Equivalences in Relational Algebra
A projection commutes with a selection that only
uses attributes retained by the projection
 Selection between attributes of the two arguments of
a cross-product converts cross-product to a join
 A selection on just attributes of R commutes with
join
R  S (i.e.,  (R  S)   (R)  S )
 Similarly, if a projection follows a join R S, we
can `push’ it by retaining only attributes of R (and S)
that are needed for the join or are kept by the
projection

H.Lu/HKUST
L04: Physical Database Design (2) -- 40
System R Optimizer
1.
2.
3.
4.
5.
6.
Find all plans for accessing each base table
For each table
• Save cheapest unordered plan
• Save cheapest plan for each interesting order
• Discard all others
Try all ways of joining pairs of 1-table plans; save cheapest
unordered + interesting ordered plans
Try all ways of joining 2-table with 1-table
Combine k-table with 1-table till you have full plan tree
At the top, to satisfy GROUP BY and ORDER BY
• Use interesting ordered plan
• Add a sort node to unordered plan
H.Lu/HKUST
L04: Physical Database Design (2) -- 41
Source: Selinger et al, “Access Path Selection in a Relational Database Management System”
H.Lu/HKUST
L04: Physical Database Design (2) -- 42
Note: Only branches for NL join are shown here. Additional branches
for other join methods (e.g. sort-merge) are not shown.
Source: Selinger et al, “Access Path Selection in a Relational Database Management System”
H.Lu/HKUST
L04: Physical Database Design (2) -- 43
What is “Cheapest”?




Need information about the relations and indexes involved
Catalogs typically contain at least:
 # tuples (NTuples) and # pages (NPages) for each relation.
 # distinct key values (NKeys) and NPages for each index.
 Index height, low/high key values (Low/High) for each
tree index.
Catalogs updated periodically.
 Updating whenever data changes is too expensive; lots of
approximation anyway, so slight inconsistency ok.
More detailed information (e.g., histograms of the values in
some field) are sometimes stored.
H.Lu/HKUST
L04: Physical Database Design (2) -- 44
Estimating Result Size
SELECT attribute list
FROM relation list
WHERE term1 AND ... AND termk

Consider a query block:

Maximum # tuples in result is the product of the cardinalities
of relations in the FROM clause.
Reduction factor (RF) associated with each termi reflects the
impact of the term in reducing result size
 Term col=value has RF 1/NKeys(I)
 Term col1=col2 has RF 1/MAX(NKeys(I1), NKeys(I2))
 Term col>value has RF (High(I)-value)/(High(I)-Low(I))
Result cardinality = Max # tuples * product of all RF’s.
 Implicit assumption that terms are independent!


H.Lu/HKUST
L04: Physical Database Design (2) -- 45
Cost Estimates for Single-Table Plans




Index I on primary key matches selection:
 Cost is Height(I)+1 for a B+ tree, about 1.2 for hash
index.
Clustered index I matching one or more selects:
 (NPages(I)+NPages(R)) * product of RF’s of matching
selects.
Non-clustered index I matching one or more selects:
 (NPages(I)+NTuples(R)) * product of RF’s of matching
selects.
Sequential scan of file:
 NPages(R).
Note: Typically, no duplicate elimination on projections!
(Exception: Done on answers if user says DISTINCT.)
H.Lu/HKUST
L04: Physical Database Design (2) -- 46
Counting the Costs



SELECT S.sname
With 5 buffers, cost of plan:
 Scan Reserves (1000) + write temp FROM Reserves R, Sailors S
T1 (10 pages, if we have 100
WHERE R.sid=S.sid AND
boats, uniform distribution)
R.bid=100 AND S.rating>5
 Scan Sailors (500) + write temp T2
(250 pages, if we have 10 ratings).
 Sort T1 (2*10*2), sort T2
(On-the-fly)
sname
(2*250*4), merge (10+250),
total=2300
 Total: 4060 page I/Os
(Sort-Merge Join)
If we used BNL join, join cost =
sid=sid
10+4*250, total cost = 2770
If we ‘push’ projections, T1 has only(Scan;
(Scan;
rating
>
5
bid=100
write to
write to
sid, T2 only sid and sname:
temp T2)
temp T1)
 T1 fits in 3 pages, cost of BNL
drops to under 250 pages, total <
Reserves
Sailors
2000
H.Lu/HKUST
L04: Physical Database Design (2) -- 47
Exercise





Reserves: 100,000 tuples, 100
tuples per page
With clustered index on bid of
Reserves, we get 100,000/100 =
1000 tuples on 1000/100 = 10
pages
Join column sid is a key for Sailors
- at most one matching tuple
Decision not to push rating>5
before the join is based on
availability of sid index on Sailors
Cost: Selection of Reserves tuples
(10 I/Os); for each tuple, must get
matching Sailors tuple (1000*1.2);
total 1210 I/Os
H.Lu/HKUST
sname
(On-the-fly)
rating > 5 (On-the-fly)
sid=sid
bid=100
(Index Nested Loops,
with pipelining )
Sailors (Use hash
Index on sid)
(Use
Reserves
clustered
index on
sid)
L04: Physical Database Design (2) -- 48
L05: Query Processing & Tuning Queries
H.Lu/HKUST

Query Processing – Principles

Tuning Queries
Avoid Redundant DISTINCT
SELECT DISTINCT ssnum
FROM Employee
WHERE dept = ‘information systems’
DISTINCT usually entails a sort operation
 Slow down query optimization because one more
“interesting” order to consider
 Remove if you know the result has no duplicates

H.Lu/HKUST
L04: Physical Database Design (2) -- 50
Change Nested Queries to Join
SELECT ssnum
FROM Employee
WHERE dept IN (SELECT dept FROM Techdept)

Might not use index on Employee.dept
SELECT ssnum
FROM Employee, Techdept
WHERE Employee.dept = Techdept.dept

Need DISTINCT if an employee might belong to multiple
departments
H.Lu/HKUST
L04: Physical Database Design (2) -- 51
Avoid Unnecessary Temp Tables
SELECT * INTO Temp
FROM Employee
WHERE salary > 40000
SELECT ssnum
FROM Temp
WHERE Temp.dept = ‘information systems’


Creating temp table causes update to catalog
Cannot use any index on original table
SELECT ssnum
FROM Employee
WHERE Employee.dept = ‘information systems’
AND salary > 40000
H.Lu/HKUST
L04: Physical Database Design (2) -- 52
Avoid Complicated Correlation Subqueries
SELECT ssnum
FROM Employee e1
WHERE salary =
(SELECT MAX(salary)
FROM Employee e2
WHERE e2.dept = e1.dept

Search all of e2 for each e1 record!
SELECT MAX(salary) as bigsalary, dept INTO Temp
FROM Employee
GROUP BY dept
SELECT ssnum
FROM Employee, Temp
WHERE salary = bigsalary
AND Employee.dept = Temp.dept
H.Lu/HKUST
L04: Physical Database Design (2) -- 53
Avoid Complicated Correlation Subqueries

> 1000
Throughput improvement percent
80
> 10000
70
60
50
SQLServer 2000
Oracle 8i
DB2 V7.1
40
30
20
10
0
correlated subquery
-10
H.Lu/HKUST
SQL Server 2000 does a good
job at handling the correlated
subqueries (a hash join is used
as opposed to a nested loop
between query blocks)
 The techniques
implemented in SQL Server
2000 are described in
“Orthogonal Optimization
of Subqueries and
Aggregates” by C.GalindoLegaria and M.Joshi,
SIGMOD 2001.
L04: Physical Database Design (2) -- 54
Join on Clustering and Integer Attributes
SELECT Employee.ssnum
FROM Employee, Student
WHERE Employee.name = Student.name


Employee is clustered on ssnum
ssnum is an integer
SELECT Employee.ssnum
FROM Employee, Student
WHERE Employee.ssnum = Student.ssnum
H.Lu/HKUST
L04: Physical Database Design (2) -- 55
Avoid HAVING when WHERE is enough
SELECT AVG(salary) as avgsalary, dept
FROM Employee
GROUP BY dept
HAVING dept = ‘information systems’

May first perform grouping for all departments!
SELECT AVG(salary) as avgsalary
FROM Employee
WHERE dept = ‘information systems’
GROUP BY dept
H.Lu/HKUST
L04: Physical Database Design (2) -- 56
Avoid Views with unnecessary Joins
CREATE VIEW Techlocation
AS SELECT ssnum, Techdept.dept, location
FROM Employee, Techdept
WHERE Employee.dept = Techdept.dept
SELECT dept
FROM Techlocation
WHERE ssnum = 4444

Join with Techdept unnecessarily
SELECT dept
FROM Employee
WHERE ssnum = 4444
H.Lu/HKUST
L04: Physical Database Design (2) -- 57
Aggregate Maintenance
Materialize an aggregate if needed “frequently”
 Use trigger to update

create trigger updateVendorOutstanding on orders for insert as
update vendorOutstanding
set amount =
(select vendorOutstanding.amount+sum(inserted.quantity*item.price)
from inserted,item
where inserted.itemnum = item.itemnum
)
where vendor = (select vendor from inserted) ;
H.Lu/HKUST
L04: Physical Database Design (2) -- 58
Avoid External Loops

No loop:
sqlStmt = “select * from lineitem where l_partkey <= 200;”
odbc->prepareStmt(sqlStmt);
odbc->execPrepared(sqlStmt);

Loop:
sqlStmt = “select * from lineitem where l_partkey = ?;”
odbc->prepareStmt(sqlStmt);
for (int i=1; i<200; i++)
{
odbc->bindParameter(1, SQL_INTEGER, i);
odbc->execPrepared(sqlStmt);
}
H.Lu/HKUST
L04: Physical Database Design (2) -- 59
throughput (records/sec)
Avoid External Loops
600
500
400
300
200
100
0
loop


no loop
SQL Server 2000 on
Windows 2000
Crossing the application
interface has a significant
impact on performance
H.Lu/HKUST
Let the DBMS optimize
set operations
L04: Physical Database Design (2) -- 60
Avoid Cursors
 No cursor
select * from employees;

Cursor
DECLARE d_cursor CURSOR FOR select * from
employees;
OPEN d_cursor
while (@@FETCH_STATUS = 0)
BEGIN
FETCH NEXT from d_cursor
END
CLOSE d_cursor
go
H.Lu/HKUST
L04: Physical Database Design (2) -- 61
Throughput (records/sec)
Avoid Cursors
5000

4000
3000

2000
1000
0
cursor
H.Lu/HKUST
SQL
SQL Server 2000 on
Windows 2000
Response time is a few
seconds with a SQL query
and more than an hour
iterating over a cursor
L04: Physical Database Design (2) -- 62
Retrieve Needed Columns Only
H.Lu/HKUST


Throughput (queries/msec)
 All
Select * from
lineitem;
 Covered subset
Select
l_orderkey,
l_partkey,
l_suppkey,
l_shipdate,
l_commitdate
from lineitem;
Avoid transferring
unnecessary data
May enable use of a
covering index.
1.75
1.5
1.25
all
covered subset
1
0.75
0.5
0.25
0
no index
index
L04: Physical Database Design (2) -- 63
Use Direct Path for Bulk Loading
sqlldr directpath=true control=load_lineitem.ctl data=E:\Data\lineitem.tbl
load data
infile "lineitem.tbl"
into table LINEITEM append
fields terminated by '|'
(
L_ORDERKEY, L_PARTKEY, L_SUPPKEY, L_LINENUMBER,
L_QUANTITY, L_EXTENDEDPRICE, L_DISCOUNT, L_TAX,
L_RETURNFLAG, L_LINESTATUS, L_SHIPDATE DATE "YYYYMM-DD", L_COMMITDATE DATE "YYYY-MM-DD",
L_RECEIPTDATE DATE "YYYY-MM-DD", L_SHIPINSTRUCT,
L_SHIPMODE, L_COMMENT
)
H.Lu/HKUST
L04: Physical Database Design (2) -- 64
Use Direct Path for Bulk Loading

Throughput (rec/sec)
50000
40000
30000
20000
10000
65
0
conventional
H.Lu/HKUST
direct path
insert
Direct path loading
bypasses the query engine
and the storage manager. It
is orders of magnitude
faster than for conventional
bulk load (commit every
100 records) and inserts
(commit for each record).
L04: Physical Database Design (2) -- 65
Some Idiosyncrasies
OR may stop the index being used
 break the query and use UNION
 Order of tables may affect join implementation

H.Lu/HKUST
L04: Physical Database Design (2) -- 66
Query Tuning Principles
Avoid redundant DISTINCT
 Change nested queries to join
 Avoid unnecessary temp tables
 Avoid complicated correlation subqueries
 Join on clustering and integer attributes
 Avoid HAVING when WHERE is enough
 Avoid views with unnecessary joins
 Maintain frequently used aggregates
 Avoid external loops

H.Lu/HKUST
L04: Physical Database Design (2) -- 67
Query Tuning Principles
Avoid cursors
 Retrieve needed columns only
 Use direct path for bulk loading

H.Lu/HKUST
L04: Physical Database Design (2) -- 68