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Aliases and Contexts Which once will give the better
performence when resolve the loops?
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Answer Posted / narender reddy
Join path: a join path is a series of joins that a query can use to access data in the tables linked by the joints. Join path problems can arise from the limited way that lookup and fact tables are related in a relational data base. The three major join problems that you encounter when designing a schematic are the following:
1. Loops
2. Chasm traps
3. Fan traps
Queries can return incorrect results due to the limitations in tha way that joins are performed in relational databases.
Loops: joins between tables form a continues path
Solved by: shortcut join(3 tables), aliases( more than 3 tables with one fact table), context( more than 3tables with more than one fact table),
Chasm traps: many to one joins from two fact tables converge on a single lookup table. This type of join convergence can lead to a join path problem called as a chasm trap.
Solved by: context, creating multiple SQL statements for each measure, creating multiple universes( webi only),
Fan traps: a one to many join links a table which is in turn linked by a one to many join. This type of serial many to one joins can lead to a join path problem called a fan trap.
Solved by: alias, using multiple SQL statements for each measure, creating the context using the alias.
Aliases: An alias is a table that is a exact duplicate of the original table, with a different name. The data in the table is exactly the same as the original table. Using aliases resolve loops and fan traps.
Context: context is a collection of joins which provide a valid query path for user to generate SQL. Using contexts we can resolve loops and chasm traps and assisting in some solutions of fan traps.
-------- when we use alias to resolve the loop we are using extra duplicate table, if we have more tables in schema it will decrease the performance so alias always decrease the performance compair to context.
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