sql - Understanding how primary key columns are included in a non-clustered index -


assume have table called 'demo' 4 columns; 'a', 'b', 'c' , 'd'. primary key clustered index 'demo' table contains columns 'a' , 'b' in order.

the 'actual execution plan' query referencing table 'demo' has suggested new non-unique non-clustered index required column 'b' , should include column 'a'.

if create non-unique non-clustered index on column 'b' need include column 'a' or part of non-clustered index because in primary key?

if primary key column 'a' part of non-clustered index, column 'a' stored include column or part of non-clustered key?

the 'actual execution plan' query referencing table 'demo' has suggested new non-unique non-clustered index required column 'b' , should include column 'a'.

...

if primary key column 'a' part of non-clustered index, column 'a' stored include column or part of non-clustered key?

in case column a presented on levels of non-clustered index part of clustered index key. index suggested non-unique needs uniquefier , clustered index key used purpose.

if offered index unique, column a stored on leaf level of index part of row locator in case of clustered table clustered index key.

column a not stored twice if include explicitly included column of index, advice include it. make difference when 1 day decides turn clustered table heap (by dropping clustered index). in case if did not include column a explicitly in non clustered index, lost , not contained in non-clustered index anymore


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