Does index work with view? - mysql

Question

Assume that I have two tables:
table1(ID, attribute1, attribute2) and
table2(ID, attribute1, attribute2) with ID is primary key of two table
and I have a view:
create view myview as
select ID, attribute1, attribute2 from table1
union
select ID, attribute1, attribute2 from table1
Can I use advantage of index of primary key (in sql in general and for mysql in my case), when I execute query like following query ?
select * from myview where ID = 100

Answer 1

It depends on your query. Using a view may limit the indexes that can be used efficiently.
For example using a table I have handy I can create a view using 2 UNIONed selects each with a WHERE clause.
CREATE VIEW fred AS
SELECT *
FROM item
WHERE code LIKE 'a%'
UNION SELECT *
FROM item
WHERE mmg_code LIKE '01%'
Both the code and the mmg_code fields have indexes. The table also has id as a primary key (highest value is about 59500).
As a query I can select from the view, or do a query similar to the view, or I can use an OR (all 3 should give the same results). I get 3 quite different EXPLAINs:-
SELECT *
FROM item
WHERE id > 59000
AND code LIKE 'a%'
UNION SELECT *
FROM item
WHERE id > 59000
AND mmg_code LIKE '01%';
gives and EXPLAIN of
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY item range PRIMARY,code,id,id_mmg_code,id_code,code_id PRIMARY 4 NULL 508 Using where
2 UNION item range PRIMARY,id,mmg_code,id_mmg_code,id_code,mmg_code_id PRIMARY 4 NULL 508 Using where
NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL Using temporary
while the following
SELECT *
FROM item
WHERE id > 59000
AND (code LIKE 'a%'
OR mmg_code LIKE '01%');
gives and EXPLAIN of
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE item range PRIMARY,code,id,mmg_code,id_mmg_code,id_code,code_id,mmg_code_id PRIMARY 4 NULL 508 Using where
and the following
SELECT *
FROM fred
WHERE id > 59000;
gives and EXPLAIN of
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 4684 Using where
2 DERIVED item range code,code_id code 34 NULL 1175 Using index condition
3 UNION item range mmg_code,mmg_code_id mmg_code 27 NULL 3509 Using index condition
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL Using temporary
As you can see as indexes have been used in the view it has affected the indexes which can be used when selecting from the view.
The best index is potentially the primary key, but the view doesn't use this.

Answer 2

"Can I use advantage of index of primary key (in sql in general and for mysql in my case), when I execute query like following query?"
MySQL will consider using indexes that have been defined on the underlying tables. However you cannot create an index on the view. Check link mysql Restrictions on Views for further explanation.
Using mysql explain on a query using the view will show the keys being considered under the "possible_keys" column.
EXPLAIN select * from myview where ID = 100;