how to query and display rows of an Index table (MySql DBMS) - mysql

Question

i am quite new to SQL world and i have searched so many blogs about indexes.
i have got two questions:
1-how to query directly from an Index table and select and display its contents?
for example suppose that table A got 2 indexes names A_index_1 and A_index_2 .i want to do something like this : select * from A.A_index_2 and display its contents and experiment with it.
2-the second questions is more complicated one.according to this link
the rows are sorted in a multi column Index and it claims that the very next column to the leftmost column is sorted as well .now suppose we have this index with columns such as this:
IDX1 on Table A : Country | Province | City | Street | Shop
and suppose the there are tons of rows which share the same country and province and city . now if we query : select * from A where Country='c' order By Province
, according to the link , because the province is adjacent to Country in the index so it is sorted as well and the sorting part of that query will be ignored(no-op). now suppose we want to query this : select * from A where Country='c' and Province='p' Order By City ,
the questions is :
in rows with the same province , is the city column in the index sorted as well?
and so will the sorting part will be ignored because of that?
this question implies to other columns in the index with same data in their previous columns!

Answer 1

I Found it my self.
1-according to this link , you can visualize the Index table and query it like this :
Visualizing an index helps in understanding what queries the index supports. You can query the database to retrieve the entries in index order
SELECT 'INDEX COLUMNS LIST'
FROM 'TABLE'
ORDER BY 'INDEX COLUMNS LIST'
If you put the index definition and table name into the query, you will get a sample from the index.
2-according to the link in a multi column index table , starting from leftmost column , the table gets ordered. and the rows which share the same value in leftmost column will be ordered based on second column from left and again those rows that share the same value in the first and second column will be ordered based on third column from left and so forth!

Answer 2

With whatever knowledge I have gathered while working with Oracle DB, I can say the following:
There's nothing like query 'from' index. You can query 'on' index.
Let's say you have a table:
Col1 VARCHAR2(10)
Col2 NUMBER
Col3 DATE
And this table is indexed on Col3. In such case, if you want to use index, your query should have a where clause which filters data on Col3
So basically, indexes help in faster querying when you are filtering results, provided ALL COLUMNS in an index are used in where clause of your query.
So following query uses index:
SELECT * FROM myTab WHERE Col3 BETWEEN TRUNC(SYSDATE -1) AND TRUNC(SYSDATE)
while following query does not:
SELECT * FROM myTab WHERE Col1 = 'Hello!'
Would like to add, indexes is a good idea only when you know what you are going to do. Too many indexes is also a bad practice. A good idea is to put index on the columns which are "FREQUENTLY" used in your filtering queries(WHERE clause of the query).
What you posted in your question, is something like a partition. Which is a logical space in the table. So if you want to query some specific set of data, you do something like:
SELECT * FROM myTab PARTITION(myPart);