What is the concept of the bitmap join index?
The concept of the bitmap join index in a sense you index a table on a column it doesn’t have right this you can use certain data warehouse but it also worked very well in normalized structures you can use the bitmap join index to denormalize the snowflake for the purposes of the query while if you’re still normalize data for your DML.
so we don’t have to deal with the inefficiencies of denormalization per diem l but we do get the benefits of do normalization for query that’s really what a bitmap join index does for you in effect it pre computes the join and that can eliminate the necessary to join at runtime you’ve got that really efficient bitmap structure right
Example of bitmap join index
here’s an example of what it looks like create bitmap index and protect PID X on MPD name so I’m indexing M on a column of depth note the syntax here but using what people sometimes call Oracle native syntax this is one of the very few cases where MC joins syntax is not yet supported every MC join doesn’t work try it and you’ll see the problems.
So you have to use the old-fashioned syntax but what we’re doing is indexing and put a column attempt right so what does that mean well if I run a query such as it’s a night to count staff and plush rejoinder up to a dynamic research oracle is doing the join so it’s scanning depth applying the filter to finding the department number of the research department it’s using that to probe my depth on ducks on amp right so what if that index didn’t exist then it’ll be even worse by drop index depth I get rid of Joe Brawley while and artists as well job then we should see something even worse and we do what we are now having to do is a merge point Oracle shows numbers showing the cost is going so if I’m to do it as a join and we’re applying filters quite late in the process
Now try it with that bitmap index they’re a bitmap join index because it’s got the syntax of the drawing embedded within it so create the index create a bitmap index on m p– d dot d name done try my query now.
bitmap join index
How cool is that course has come down to one and I haven’t had to hit either table that’s quite phenomenal I haven’t had to hit either table to run that query and this is not just meaning that the query is going to run well for me it also means this all this workload table access for in next full scan and that will have been in direct center buffer cache that will have been hammering the buffer cache with single block reads that may well have been direct read it’s just incredibly efficient in comparison incredibly efficient jaundice yeah does the bitmap joins suffer the same DML limitations or issues if you are yes it does you do have to bear in mind the possibility of locking the issue we’ve got here I don’t if I’ll actually be able to demonstrate it but I can try the issue we’ve got is that normal locking is done in the black feathers in the header of each block
You have interested transaction lists and so on locking information
And you can space in the base of the block header says these rows or locks that are straightforward the problem we have with a bitmap index isn’t the bitmap index is only bits it isn’t really rowing aware so you don’t get row-level locking
So if I do this I may well have a problem if some DMLs let’s see if I can actually generate the problem in one row I will well it’s just select star from M right so they’re my 14 rows set what a trace right I’ll delete me.
Smith at the top here one row deleted and in another session, we’ll see if we can set up the problem and also I’ll do is delete in this window I’ll delete Mr. Miller look at that is hang that’s the problem it bitmap index is that’s the issue why is that session hanged the clue my index is on job Smith his job is Clark Miller guess what his job is Clark it’s a matter of luck you might not hit this problem but because of the structure through bad luck and the way the bitmaps are stored in blocks.
You have to lock groups of rows at a time of course if it’s a huge table there’s a pretty good chance this won’t happen no but it might once I roll back that or commit us then the other session comes free night so that’s what you’ve got to hang on so there may be issues with concurrent DML maintenance fit map index should we rebuild coalesce what you’d what your take my fake is that you never rebuilds in next unless you have proven that you need to and I take that approach for both b-tree and bitmap the maintenance all depends what release your arm of course but this comes up, again and again, doesn’t this oracles improve the algorithms for index maintenance hugely in recent releases and the idea that you need frequent rebuilds that is dreadfully 20th century the business of navigating a normalized structure right I’ll go to the HR schema.
Here connects HR / HR remember
how I described the snowflake each employee and departments in a location is linked country and the country is in a region all right consider a query that you might want to run in that environment so here’s my query think it’s a tile from employees where region name equals Europe
I want to see how many Europeans our employee will skip from employees to Europe I have to navigate that normalized structure I have to go from employees departments from departments and locations from locations to countries from countries to regions and only when I get down there can I apply the filter and you see that in execution plan the filter full scan of regions in four rows that’s where we can apply the filter and yes I could index excess but I’ve had to navigate that whole normalized structure to get there and it’s been costed or seven right bitmaps join indexes we can in effect D normalizes for the purposes of this query.
So the syntax looks like this create bitmap index and purge picks on employees our region name and then we have the join conditions so in my bitmap index creation statement and navigating the normalized structure so I’m indexing employees on a column from a table for levels out index will see follicles going to use it it may refuse to drop the index let’s create them again we’ll see follicle uses the index it may not do so but let’s have a look well Oracle isn’t to use the index yes we’ve got the same plan this on seven there could be any number which is an adaptive plan there could be any number of reasons for this so I’m just going to put in a hint so I’ve hinted use that index bingo look at that so that’s horrendously complicated execution plan cost at seven navigating the full structure is now reduced with this cost at one and I haven’t actually had to hurt any of the tables that’s phenomenal you.
please read next article also
You have to wait 15 seconds.
(tutelugu.co)
Super
R4AMJOTUU11S