I have observed a scope of considerable performance improvement in-case of index by a very minor code change.
Consider the below schema:
create table tbl2(id1 int, id2 varchar(10), id3 int);
create index idx2 on tbl2(id2, id3);
Query as:
select count(*) from tbl2 where id2>'a' and id3>990000;
As per current design, it takes following steps to retrieve index tuples:
1. Find the scan start position by searching first position in BTree as per the first key condition i.e. as per id2>'a'
2. Then it fetches each tuples from position found in step-1.
3. For each tuple, it matches all scan key condition, in our example it matches both scan key condition.
4. If condition match, it returns the tuple otherwise scan stops.
Now problem is here that already first scan key condition is matched to find the scan start position (Step-1), so it is obvious that any further tuple also will match the first scan key condition (as records are sorted).
So comparison on first scan key condition again in step-3 seems to be redundant.
So my proposal is to skip the condition check on the first scan key condition for every tuple.
I have prototype code to see the result, Below are my some test data as per the query attached with this mail:
Time:
Original Code took: 3621 ms
Optimized Code took: 2576 ms
So overall performance improvement is around 29%.
Instruction:
Original Code took: 20057
Optimized Code took: 14557
So overall instruction improvement is around 27%.
Also I observed that only for function varstr_cmp, around 661M instruction was taken, which is completely saved for optimized code.
This optimization can be extended:
1. For a case where once all scan key matches the condition, no need to check condition for further tuples for any scan key. Just keep returning the tuple.
2. Above approach can be used for ‘<’ operator also by finding the position of last matching tuples.
I would like to submit the patch for this improvement.
Please provide your feedback. Also let me know if I am missing something.
Thanks and Regards,
Kumar Rajeev Rastogi