Database Technologies and Applications Week 5

UNION- R1+R2= R3 w/ tuples in either. Attrib must be defined on same domain

DIFFERENCE- diff of realtions R1 and R2 is new R3. Tuples only in R1

RESTRICTION- extract specified tuples from spc relation R1 by imposing cond on relationRESULT R2 has only spc tuples (replaced by selected in new systesm)

PRODUCT- R3 w/ concatenation of every tuple in R1 wit every tuple R2

PROJECTION- extract spc attri from spc R1 into new R2

JOIN- R1+R2= R3 w/ tuples R1/2 satisfy spc condition (natch)

INTERSECTION- two spc relations R1/2 where R3 has tuple in R1/2

DIVISION- relation 

PRIMARY OPERTAIONS- union, difference, select, project, adn product

SECONDARY OPERATIONS- join, intersect, divide

Aliasing-when you need to refer to different tuples in the same relations

Extend- takes realtion as input and reutrns with add column

Aggregate- summerize values of a column of realtion

SQL is ex of DSL develp in 70s

commands in command prompts of embedded applicatoins

advantages- rapid software devl, higher quality, higher productivity, data independence, standardization

Web Engines Part One

Web engines operate with large and geographically distributed centers. Clusters or ind, data centers has specific jobs, such as crawling. Large scale replication required.

CRAWLING

Speed, politeness, excluded/dupped content, spam rejection

Indexing

 inverted file to rapidly identify indexing terms

 first phase, scanning, the indexer scans the text of each input document

 for high-quality rankings, real indexers store additional information in the postings

TERM LOOKUP, COMPRESSION, PHRASES, ANCHOR TEXT,