I am confused with these 3 terms: cover index, compound index and multi-fields index.
Are they same things? or each of them has subtle difference?
Thanks
Compound index and multi-fields Index are the same. Other terms for the same are Multi-Column Index and Concatenated Index.
That are indexes that contain more than one column.
I suspect that Cover Index is actually Covering Index which is something entirely different, better described as Index-Only-Scan.
That is not a property of an index, it is describing how the index is used. It means that a particular query can be satisfied with data from the index only, not needing to read the table data. (Note: An index copies data from the table).
A single index can be a covering-index for one query, but not cover another query (that accesses columns not included (covered) in the index). Think of "The index covers the entire query."
More about database indexing: http://use-the-index-luke.com/
Related
I'm studying about indexes in a database and I'm having some ambiguity. In a non-clustered index, we learned that the datafiles are not sorted and that each index entry points to a record in the datafile. However, the secondary index doesn't seem to be any different from this one. Can the two be considered the same? If not, I'd appreciate it if you could let me know which one is different.
I am trying to google it from couple hours and it still is not clear for me.
What is the difference between:
Create Index NonClusteredComposit_IDX ON Table(id,quantity,price)
Create Index NonClusteredCompositAndInclude_IDX ON Table(id) Include (price,quantity).
On the Index lvl only.
I understand how they work and even when to use them.
But that I can't understand is that how data is stored inside the NonClusteredCompositAndInclude_IDX?
What would change on this schema where:
Index page contains Indexed data (id,quantity,price) and pointer to RID (when a table is a heap) or pointer to a page in B-tree (for B-tree/Clustered tables).
From the documentation, I know that when I include columns then data are stored in Leaf node but I don't see any difference between this and normal Index On(1,2,3) if we are talking about architecture inside Index.
Can anyone can describe me differences in index architecture?
Thanks!
In first approach, sorting will be on these three attributes - id,quantity,price
In second approach, sorting will be on "id" only but that "id" contains values of "quantity,price" hence it does not require to do key lookup or rid lookup to get the respective attributes.
To illustrate this if you create below indexes in one of the tables, both does Index seek but if you check the number of rows read, it differs as one takes from sorted data and the 2nd approach does full scan for selected "id"
On checking Number of reads for the first index...
On checking Number of reads for the 2nd index it proves that it does full scan of index for the seek'd data, hence you get 188 records
I googled this a lot many times but I didn't get the exact explanation for the same.
I am working on a complex database structures (in Oracle 10g) where I hardly have a primary key on one single column except for the static tables.
Now my question is consider a composite primary key ID (LXI, VCODE, IVID, GHID). Since it's a primary key, Oracle will provide a default index.
Will I get ONE (system generated) single index for the primary key itself or for its sub-columns also?
Asking this because I am retrieving data (around millions of records) based on individual columns as well. Now if system generates the indices for the individual columns as well. Why my query runs pretty faster than how it actually runs when I explicitly define indices for each individual column.
Please give a satisfactory answer
Thanks in advance
A primary key is a non-NULL unique key. In your case, the unique index has four columns, LXI, VCODE, IVID GHID in the order of declaration.
If you have a condition on VCODE but not on LXI, then most databases would not use the index. Oracle has a special type of index scan called the "skip scan", which allows for this very situation. It is described in the documentation.
I would expect an index skip scan to be a bit slower than an index range scan on individual columns. However, which is better might also depend on the complexity of the where clause. For instance, three equality conditions on VCODE, IVID and GHID connected by AND might be a great example for the skip scan. And, such an index would cover the WHERE clause -- a great efficiency -- and better than one-column indexes.
As a note: index skip scans were introduced in Oracle 9i, so they are available in Oracle 10.
It will not generate index for individual column. it will generate a composite index
first it will index on LXI
then next column like that it will be a tree structure.
if you search on 1st column of primary key it will use index to use index for second you have to combine it with the first column
ex : select where ...LXI=? will use index PK
select where LXI=? and VCODE=? alse use pk
but select where VCODE=? will not use it (without LXI)
Looking at the missing index DMVs on SQLServer it suggests I add the following index:
CREATE INDEX [IXFoo] ON [a].[b].[MyTable] ([BarFlag]) INCLUDE ([BazID])
There's two things that confuse me.
[BarFlag] is a bit field. Hardly highly selective, why put an index on a bit field?
Why not use a composite index in this case.: CREATE INDEX [IXFoo] ON [a].[b].[MyTable] ([BarFlag],[BazID])
I guess I'm not understanding the INCLUDE keyword properly. I've looked at msdn for an explanation but I'm still unclear.
Can someone explain why this index is suggested over a composite and explain the INCLUDE keyword to me?
The main difference is this:
if you create a composite index on (BarFlag, BazID), then your index will contain both values on all levels of the index b-tree; this means, the query analyzer will also have the chance to use both values when making decisions, and this can support queries that specify both columns in a WHERE clause
if you create an index on (BarFlag) and only include (BazID), then your index will contain only BarFlag values on all levels of the index b-tree, and only on the leaf level, the "last" level, there will also be the values of BazID included. The BazID values cannot be used in selecting the data - they're just present at the index leaf level for lookup.
Just for an INT and a BIT that isn't much of a concern, but if you're dealing with a VARCHAR(2000) column, you cannot add that to the actual index (max. is 900 byte per entry) - but you can include it.
Having a column included in an index can be useful if you select for these two values - then if SQL Server finds a match for BarFlag, it can look up the corresponding BazID value in the leaf-level node of the index itself and it can save itself a trip back to the actual data page (a "bookmark lookup") to go grab that value from the data pages. This can be a massive boost for performance
And you're right - having an index just on BarFlag (BIT) really doesn't make sense - then again, that DMV only suggests indices - you're not supposed to blindly follow all its recommendations - you still need to think and consider if those are good recommendations (or not).
The INCLUDE keyword just means that the value of the included columns should be stored in the index itself so that for queries like the following:
SELECT BazID FROM MyTable WHERE BarFlag = #SomeValue
It isn't necessary to do an additional lookup on the table itself in order to find the value of BazID after doing an index seek.
So here I am looking at this huge oracle 10g table. I looked at its indexes and see that ALL of the columns are under one unique index. Does this provide actually provide any performance benefits?
Possibly, possibly not. It could be that the unique index is implementing a constraint to ensure that rows are indeed unique, and is not intended to help with performance at all. There could be a performance benefit for indexed lookup queries, because they won't need to access the actual table at all.
On the face of it it sounds like this should have been created as an INDEX ORGANIZED table.
From a performance standpoint, I would say that having all fields in a single index (unique or not) is generally not a good idea.
Any update to the table will result in that index being updated. In a "normal" table (the word normal used very loosely), there will be some fields not indexed. If one of those fields is updated, then it is more efficient because no indexes need to be updated.
The described index is somewhat limited in optimization of queries. For example, suppose the table has fields a, b, c, d, and e and that the index is defined with the fields in that order. Any query that does not reference a in the WHERE clause cannot use that index.
Depending on the number and size of fields involved, such an index could possibly have very large keys. With larger keys, it means that fewer keys can be stored in each page, and so an update to the index means that more page reads and writes will be involved.