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Merge Join
--------------------
Merge join is a concept in which rows to be joined must be
present in same AMP. If the rows to be
joined are not on the same AMP, Teradata will either
redistribute the data or duplicate the data inspool to make that
happen based on row hash of the columns involved in the joins WHERE
Clause.
If two tables to be joined have same primary Index, then the
records will be present in Same
AMP and Re-Distribution of records is not required.
There are four scenarios in which redistribution can happen for
Merge Join
Case 1:If joining columns are on UPI = UPI, the records to be
joined are present in Same AMP and
redistribution is not required. This is most efficient and
fastest join strategy
Case 2:If joining columns are on UPI= Non Index column, the
records in 2nd table has to be
redistributed on AMP's based on data corresponding to first
table.
Case 3:If joining columns are on Non Index column= Non Index
column, the both the tables are
to be redistributed so that matching data lies on same amp , so
the join can happen on redistributeddata. This strategy is time
consuming since complete redistribution of both the tables takes
across all
the amps
Case 4:For join happening on Primary Index, If the Referenced
table (second table in the join) is very
small, then this table is duplicated /copied on to every
AMP.
Nested Join
-------------------
Nested Join is one of the most precise join plans suggested by
Optimizer .Nested Join works on
UPI/USI used in Join statement and is used to retrieve the
single row from first table . It then checks
for one more matching rows in second table based on being used
in the join using an index (primary
or secondary) and returns the matching results.
Example:
SelectEMP.Ename, DEP.Deptno, EMP.salary
from
EMPLOYEEEMP,
DEPARTMENTDEP
WhereEMP.Enum= DEP.Enum
andEMp.Enum= 2345; -- this results in nested join
Hash join----------------
Hash join is one of the plans suggested by Optimizer based on
joining conditions. We can say Hash
Join to be close relative of Merge based on its functionality.
In case of merge join, joining would
happen in same amp. In Hash Join, one or both tables which are
on same amp are fit completely
inside the AMP's Memory . Amp chooses to hold small tables in
its memory for joins happening on
ROW hash.
Advantages of Hash joins are
1. They are faster than Merge joins since the large table doesnt
need to be sorted.
2. Since the join happening b/w table in AMP memory and table in
unsorted spool, it happens so
quickly.
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Exclusion Join
-------------------------
These type of joins are suggested by optimizer when following
are used in the queries
- NOT IN
- EXCEPT- MINUS
- SET subtraction operations
SelectEMP.Ename, DEP.Deptno, EMP.salary
from
EMPLOYEEEMP
WHEREEMP.EnumNOTIN
( SelectEnumfrom
DEPARTMENTDEP
whereEnumisNOTNULL);
Please make sure to add an additional WHERE filter with IS NOT
NULL since usage of
NULL in a NOT IN list will return no results.
Exclusion join for following NOT In query has 3 scenarios
Case 1:matched data in "NOT IN" sub Query will disqualify that
row
Case 2:Non-matched data in "NOT IN" sub Query will qualify that
row
Case 3:Any Unknown result in "NOT IN" will disqualify that row -
('NULL' is a typical example of this
scenario).
