sql - Details of impact for indexes, primary keys, unique keys -

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i think know enought theory, have little experience optimizing db in real world. know points of view, thoughts or experiences.

let's imagine scenario like:

table key: c1, c2, c3, c4 index: c7, c3, c2

table b key: c1, c2, c3, c4 index: c1, c5

all non-clustered. tables have 40+ fields. feeded daily night , have updates during day.

table a, if more queries benefit key index, index impact negatively? because insert/delete has update 2 indexes instead of 1.

table b, has field @ index, not present in key.

could query using c1, c5

benefit key?: key: c1, c2, c3, c4, c5

so index droped.

what impact order of fields has? key: c1, c2, c3 key: c3, c1, c2

a typical scenario me process_date, client_number, operation. , feeds bunch of data each day (process_date).

if more queries benefit key index, index impact negatively? because insert/delete has update 2 indexes instead of 1.

a non-clustered index has negative impact on insert/update/delete performance. negative impact outweighed increased performance of selects.

could query using c1, c5 benefit key?: key: c1, c2, c3, c4, c5

yes, if few rows share same c1, index effective.

what impact order of fields has? key: c1, c2, c3 key: c3, c1, c2

order important, both filtering , ordering. index on (c1,c2) can used where c1 = 1 , where c1 = 1 , c2 = 1, not where c2 = 1. likewise, helps order c1, not order c2.


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