Hi,
1 What is the technical structure of index?
For example, I have a table ztable with 5 fields, F1(key), F2(key). F3. F4.F5. If I create a index on F3,F4, the system will create a copy of this table. Is the structure of this copy as follow?
F3, F4, pointer(for the lineno of the recorder)
So this copy will have fewer lines than the ZTABLE and is sorted. If the DB optimizer choose this index in selection, it will be fast for the 2 reasons?
2 By what is the sequence of the fields in the index determined? I read a BLOG about permance, the author make a example with DB05, and say if the distinct values of certain field is between 1-1000 lines, it is good to set a secondary index for the field.
I wonder, if the F3 has only 5 distinct value for all the data in the ZTABLE, and the F4 has 100 distinct value. Which one should be the first field in the index and why?
You seem to be correct for some but not all.
Struture of index will
for unique key index F3, F4, row id of database
it could have only one row id when your where condition match with the index then it goes to only index table first and read the row id and read record directly from the row id.
But for non unique key table store all the F3, F4, row id of record which math value of F1 and F4. It can have multiple value.
When your where condition math with condition partially ie left part of the key or whole part key then read all match record from the index table and then read the database physical table and check for addation where condition if any.
Index is determine on basis of the where condtion which is most left part of index will match.
if your where condition F4 and in your index has f3 f4 then index will not consider, It create gap between index condition and read whole table database. But if you consider f3 then only it consider the index to read because it match the left most part with index fields.
For more details How to create secondary index ? Search for blog in sdn I reply many time.
Bluesky
