mysql - understanding indexs with a simple select query -

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mysql - understanding indexs with a simple select query -

i'm trying understand how work indexes basic queries.

example:

i have table 'testme' next columns:

id int primary key username varchar(20) data1 int data2 int data3 int data_order int

if

select username,data1,data2 testme data3=5 order data_order;

which kind of indexes can utilize speed query ?

i tried adding index on clumns data3 , data_order result of 'explain' query shows doesn't utilize index.

update: using mysql cluster (ndb)

think of index 2 things... 1. order in info stored 2. quick way lookup specific pieces of info (like book index)

in example, having index on (data3, data_order) create easy find info want, , have in right order.

it still needs go table after searching index, fields username, data1, data2. reason may include them in index well. makes index bigger, using more space , more effort update. cost means index isn't beingness joined table. it's reading index.

mysql sql indexing query-optimization

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