We are structuring a project where some tables will have many records, and we intend to use 4 numeric foreign keys and 1 numeric primary, our assumption is that if we create an index for each foreign key and the default index of the primary key, the postgres planning would use all the starts (5 in total) to perform the query.
95% of the time the queries would be providing at least the 4 foreign keys.
Would each index be used to position the search faster in the sequential section of records?
Would having 4 indexes increase the speed of the query or would it suffice with a single index of the parent level (branch_id)?
Thank you for your time and experience.
example: if all foreign keys have an index
SELECT * FROM products WHERE
account_d=1 AND
organization_id=2 AND
business_id=3 AND
branch_id=4 AND
product_id=5;
example: if I only indicate the id of the primary key
SELECT * FROM products WHERE product_id=5;
If all 4 columns are specified by equality, it is possible to combine the single-column indexes using BitmapAnd. However, this would be less efficient than using one multi-column index on all four columns.
Since that will apparently be a very common query, it would make sense to have that multi-column index.
Usually you will want to index each foreign key column. Otherwise, if you want to delete an organization, for example, it would need to scan the whole table to verify that no records were still referencing it. Whichever column is the first one in the multi-column index will not need to also have a single-column index for it. But the other 3 which are not first probably still need their own indexes.
Indexes are (predominantly) used when filtering or joining tables, so whether the indexes you are proposing are useful is entirely dependent on the SQL you are running and whether the query optimiser determines that using an index would be beneficial.
For example, if you ran SELECT * FROM TABLE then none of the indexes would be used.
I can’t comment on Postgresql specifically but many/most DBMSs automatically create indexes when you define PKs/FKs - so you will get the indexes anyway, regardless of any performance tuning you are trying to implement
Update
Having individual indexes on each column is not going to help with the query you’ve provided, the optimiser will only use one of them, probably the PK. A compound index on multiple columns would help, but the more columns you add to the index, the more restrictive the pattern of queries it will benefit.
Say you have 3 columns A, B, C and include them all in WHERE clause, then having a compound index of A+B+C would be highly beneficial.
If you keep this index but your WHERE clause only has columns A, B it will still benefit significantly as the query can still use the A+B subset of the index.
If your WHERE clause only has columns A,C then it would benefit only slightly, as it would select all records from the index that start with the A value - but then would have to filter them to find the subset with the C value
Related
Imagine I have a table with the following columns:
Column: A (numer(10)) (PK)
Column: B (numer(10))
Column: C (numer(10))
CREATE TABLE schema_name.table_name (
column_a number(10) primary_key,
column_b number(10) ,
column_c number(10)
);
Column A is my PK.
Imagine my application now has a flow that queries by B and C. Something like:
SELECT * FROM SCHEMA.TABLE WHERE B=30 AND C=99
If I create an index only using the Column B, this will already improve my query right?
The strategy behind this query would benefit from the index on column B?
Q1 - If so, why should I create an index with those two columns?
Q2 - If I decided to create an index with B and C, If I query selecting only B, would this one be affected by the index?
The simple answers to your questions.
For this query:
SELECT *
FROM SCHEMA.TABLE
WHERE B = 30 AND C = 99;
The optimal index either (B, C) or (C, B). The order does matter because the two comparisons are =.
An index on either column can be used, but all the matching values will need to be scanned to compare to the second value.
If you have an index on (B, C), then this can be used for a query on WHERE B = 30. Oracle also implements a skip-scan optimization, so it is possible that the index could also be used for WHERE C = 99 -- but it probably would not be.
I think the documentation for MySQL has a good introduction to multi-column indexes. It doesn't cover the skip-scan but is otherwise quite applicable to Oracle.
Short answer: always check the real performance, not theoretical. It means, that my answer requires verification at real database.
Inside SQL (Oracle, Postgre, MsSql, etc.) the Primary Key is used for at least two purposes:
Ordering of rows (e.g. if PK is incremented only then all values will be appended)
Link to row. It means that if you have any extra index, it will contain whole PK to have ability to jump from additional index to other rows.
If I create an index only using the Column B, this will already improve my query right?
The strategy behind this query would benefit from the index on column B?
It depends. If your table is too small, Oracle can do just full scan of it. For large table Oracle can (and will do in common scenario) use index for column B and next do range scan. In this case Oracle check all values with B=30. Therefore, if you can only one row with B=30 then you can achieve good performance. If you have millions of such rows, Oracle will need to do million of reads. Oracle can get this information via statistic.
Q1 - If so, why should I create an index with those two columns?
It is needed to direct access to row. In this case Oracle requires just few jumps to find your row. Moreover, you can apply unique modifier to help Oracle. Then it will know, that not more than single row will be returned.
However if your table has other columns, real execution plan will include access to PK (to retrieve other rows).
If I decided to create an index with B and C, If I query selecting only B, would this one be affected by the index?
Yes. Please check the details here. If index have several columns, than Oracle will sort them according to column ordering. E.g. if you create index with columns B, C then Oracle will able to use it to retrieve values like "B=30", e.g. when you restricted only B.
Well, it all depends.
If that table is tiny, you won't see any benefit regardless any indexes you might create - it is just too small and Oracle returns data immediately.
If the table is huge, then it depends on column's selectivity. There's no guarantee that Oracle will ever use that index. If optimizer decides (upon information it has - don't forget to regularly collect statistics!) that the index should not be used, then you created it in vain (though, you can choose to use a hint, but - unless you know what you're doing, don't do it).
How will you know what's going on? See the explain plan.
But, generally speaking, yes - indexes help.
Q1 - If so, why should I create an index with those two columns?
Which "two columns"? A? If it is a primary key column, Oracle automatically creates an index, you don't have to do that.
Q2 - If I decided to create an index with B and C, If I query selecting only B, would this one be affected by the index?
If you are talking about a composite index (containing both B and C columns, respectively), and if query uses B column, then yes - index will (OK, might be used). But, if query uses only column C, then this index will be completely useless.
In spite of this question being answered and one answer being accepted already, I'll just throw in some more information :-)
An index is an offer to the DBMS that it can use to access data quicker in some situations. Whether it actually uses the index is a decision made by the DBMS.
Oracle has a built-in optimizer that looks at the query and tries to find the best execution plan to get the results you are after.
Let's say that 90% of all rows have B = 30 AND C = 99. Why then should Oracle laboriously walk through the index only to have to access almost every row in the table at last? So, even with an index on both columns, Oracle may decide not to use the index at all and even perform the query faster because of the decision against the index.
Now to the questions:
If I create an index only using the Column B, this will already improve my query right?
It may. If Oracle thinks that B = 30 reduces the rows it will have to read from the table imensely, it will.
If so, why should I create an index with those two columns?
If the combination of B = 30 AND C = 99 limits the rows to read from the table further, it's a good idea to use this index instead.
If I decided to create an index with B and C, If I query selecting only B, would this one be affected by the index?
If the index is on (B, C), i.e. B first, then Oracle may find it useful, yes. In the extreme case that there are only the two columns in the table, that would even be a covering index (i.e. containing all columns accessed in the query) and the DBMS wouldn't have to read any table row, as all the information is already in the index itself. If the index is (C, B), i.e. C first, it is quite unlikely that the index would be used. In some edge-case situations, Oracle might do so, though.
What is the difference between creating an index using INCLUDE function vs not?
What would be the difference between the following two indexes?
CREATE NONCLUSTERED INDEX SomeName ON SomeTable (
ColumnA
,ColumnB
,ColumnC
,ColumnD
) INCLUDE (
ColumnE
,ColumnF
,ColumnG
)
vs
CREATE INDEX SomeName ON SomeTable (
ColumnA
,ColumnB
,ColumnC
,ColumnD
,ColumnE
,ColumnF
,ColumnG
)
The INCLUDE clause adds the data at the lowest/leaf level, rather than in the index tree. This makes the index smaller because it's not part of the tree.
INCLUDE columns are not key columns in the index, so they are not ordered. This means it isn't really useful for predicates, sorting etc.. However, it may be useful if you have a residual lookup in a few rows from the key columns.
INCLUDE columns are not key columns in the index, so they are not ordered. This makes them not typically useful for JOINs or sorting. And because they are not key columns, they don't sit in the whole B-tree structure like key columns
By adding Include (or nonkey)columns, you can create nonclustered indexes that cover more queries. This is because the nonkey columns have the following benefits:
They can be data types not allowed as index key columns.
They are not considered by the Database Engine when calculating the number of index key columns or index key size.
An index with Included columns can significantly improve query performance when all columns in the query are included in the index either as key or nonkey columns. Performance gains are achieved because the query optimizer can locate all the column values within the index; table or clustered index data is not accessed resulting in fewer disk I/O operations.
For more info refer Microsoft docs: Create Indexes with Included Columns
When an execution plan uses an index, it has access to all the columns in the index. If all the columns from a given table are in the index, there is no need to refer to the original data pages. Eliminating that data page lookup is a gain in efficiency.
However, including columns in indexes has overhead for the indexing structure itself (this is in addition to duplicating the values).
The INCLUDE keyword allows for column values to be in the index, without incurring the overhead of the additional indexing structure. The purpose is to resolve queries without having to look up the column information on the original data pages.
when I create an index on a db2, for example with the following code:
CREATE INDEX T_IDX ON T(
A,
B)
is it a composite index?
if not: how can I then create a composite index?
if yes: in order to have two different index should I create them separately as:
CREATE INDEX T1_IDX ON T(A)
CREATE INDEX T2_IDX ON T(A)
EDIT: this discussion is not going in the direction I expect (but in a better one :)) I actually asked how, and not why to create separate indexes, I planed to do that in a different question, but since you anticipated me:
suppose I have a table T(A,B,C) and a search function search() that select from the table using any of the following method
WHERE A = x
WHERE B = x
WHERE C = x
WHERE A = x AND B=y (and so on AC, CB, ABC)
if I create a compose index ABC, is it going to working for example when I select on just C?
the table is quite big, and the insert\update not so frequent
Yep multiple fields on create index = composite by definition: Specify two or more column names to create a composite index.
Understanding when to use composite indexes appears to be your last question...
If all columns selected by a query are in a composite index, then the dbengine can return these values from the index without accessing the table. so you have faster seek time.
However if one or the other are used in queries, then creating individual indexes will serve you best. It depends on the types of queries executed and what values they contain/filter/join.
If you sometimes have one, the other, or both, then creating all 3 indexes is a possibility as well. But keep in mind each additional index increases the amount of time it takes to insert, update or delete, so on highly maintained tables, more indexes are generally bad since the overhead to maintain the indexes effects performance.
The index on A, B is a composite index, and can be used to seek on just A or a seek on A with B or for a general scan, of course.
There is usually not much of a point in having an index on A, B and an index on just A, since a partial search on A, B can be used if you only have A. That wider index will be a little less efficient, however, so if the A lookup is extremely frequent and the write requirements mean that it is acceptable to update the extra index, it could be justifiable.
Having an index on B may be necessary, since the A, B index is not very suitable for searches based on B only.
First Answer: YES
CREATE INDEX JOB_BY_DPT
ON EMPLOYEE (WORKDEPT, JOB)
Second Answer:
It depends on your query; if most of the time your query referrence a single column in where clause like select * from T where A = 'something' then a single index would be what you want but if both column A and B get referrenced then you should go for creating a composite one.
For further referrence please check
http://publib.boulder.ibm.com/infocenter/db2luw/v8/index.jsp?topic=/com.ibm.db2.udb.doc/admin/r0000919.htm
I am not db guy. But I need to create tables and do CRUD operations on them. I get confused should I create the index on all columns by default
or not? Here is my understanding which I consider while creating index.
Index basically contains the memory location range ( starting memory location where first value is stored to end memory location where last value is
stored). So when we insert any value in table index for column needs to be updated as it has got one more value but update of column
value wont have any impact on index value. Right? So bottom line is when my column is used in join between two tables we should consider
creating index on column used in join but all other columns can be skipped because if we create index on them it will involve extra cost of
updating index value when new value is inserted in column.Right?
Consider this scenario where table mytable contains two three columns i.e col1,col2,col3. Now we fire this query
select col1,col2 from mytable
Now there are two cases here. In first case we create the index on col1 and col2. In second case we don't create any index.** As per my understanding
case 1 will be faster than case2 because in case 1 we oracle can quickly find column memory location. So here I have not used any join columns but
still index is helping here. So should I consider creating index here or not?**
What if in the same scenario above if we fire
select * from mytable
instead of
select col1,col2 from mytable
Will index help here?
Don't create Indexes in every column! It will slow things down on insert/delete/update operations.
As a simple reminder, you can create an index in columns that are common in WHERE, ORDER BY and GROUP BY clauses. You may consider adding an index in colums that are used to relate other tables (through a JOIN, for example)
Example:
SELECT col1,col2,col3 FROM my_table WHERE col2=1
Here, creating an index on col2 would help this query a lot.
Also, consider index selectivity. Simply put, create index on values that has a "big domain", i.e. Ids, names, etc. Don't create them on Male/Female columns.
but update of column value wont have any impact on index value. Right?
No. Updating an indexed column will have an impact. The Oracle 11g performance manual states that:
UPDATE statements that modify indexed columns and INSERT and DELETE
statements that modify indexed tables take longer than if there were
no index. Such SQL statements must modify data in indexes and data in
tables. They also create additional undo and redo.
So bottom line is when my column is used in join between two tables we should consider creating index on column used in join but all other columns can be skipped because if we create index on them it will involve extra cost of updating index value when new value is inserted in column. Right?
Not just Inserts but any other Data Manipulation Language statement.
Consider this scenario . . . Will index help here?
With regards to this last paragraph, why not build some test cases with representative data volumes so that you prove or disprove your assumptions about which columns you should index?
In the specific scenario you give, there is no WHERE clause, so a table scan is going to be used or the index scan will be used, but you're only dropping one column, so the performance might not be that different. In the second scenario, the index shouldn't be used, since it isn't covering and there is no WHERE clause. If there were a WHERE clause, the index could allow the filtering to reduce the number of rows which need to be looked up to get the missing column.
Oracle has a number of different tables, including heap or index organized tables.
If an index is covering, it is more likely to be used, especially when selective. But note that an index organized table is not better than a covering index on a heap when there are constraints in the WHERE clause and far fewer columns in the covering index than in the base table.
Creating indexes with more columns than are actually used only helps if they are more likely to make the index covering, but adding all the columns would be similar to an index organized table. Note that Oracle does not have the equivalent of SQL Server's INCLUDE (COLUMN) which can be used to make indexes more covering (it's effectively making an additional clustered index of only a subset of the columns - useful if you want an index to be unique but also add some data which you don't want to be considered in the uniqueness but helps to make it covering for more queries)
You need to look at your plans and then determine if indexes will help things. And then look at the plans afterwards to see if they made a difference.
I've just heard the term covered index in some database discussion - what does it mean?
A covering index is an index that contains all of, and possibly more, the columns you need for your query.
For instance, this:
SELECT *
FROM tablename
WHERE criteria
will typically use indexes to speed up the resolution of which rows to retrieve using criteria, but then it will go to the full table to retrieve the rows.
However, if the index contained the columns column1, column2 and column3, then this sql:
SELECT column1, column2
FROM tablename
WHERE criteria
and, provided that particular index could be used to speed up the resolution of which rows to retrieve, the index already contains the values of the columns you're interested in, so it won't have to go to the table to retrieve the rows, but can produce the results directly from the index.
This can also be used if you see that a typical query uses 1-2 columns to resolve which rows, and then typically adds another 1-2 columns, it could be beneficial to append those extra columns (if they're the same all over) to the index, so that the query processor can get everything from the index itself.
Here's an article: Index Covering Boosts SQL Server Query Performance on the subject.
Covering index is just an ordinary index. It's called "covering" if it can satisfy query without necessity to analyze data.
example:
CREATE TABLE MyTable
(
ID INT IDENTITY PRIMARY KEY,
Foo INT
)
CREATE NONCLUSTERED INDEX index1 ON MyTable(ID, Foo)
SELECT ID, Foo FROM MyTable -- All requested data are covered by index
This is one of the fastest methods to retrieve data from SQL server.
Covering indexes are indexes which "cover" all columns needed from a specific table, removing the need to access the physical table at all for a given query/ operation.
Since the index contains the desired columns (or a superset of them), table access can be replaced with an index lookup or scan -- which is generally much faster.
Columns to cover:
parameterized or static conditions; columns restricted by a parameterized or constant condition.
join columns; columns dynamically used for joining
selected columns; to answer selected values.
While covering indexes can often provide good benefit for retrieval, they do add somewhat to insert/ update overhead; due to the need to write extra or larger index rows on every update.
Covering indexes for Joined Queries
Covering indexes are probably most valuable as a performance technique for joined queries. This is because joined queries are more costly & more likely then single-table retrievals to suffer high cost performance problems.
in a joined query, covering indexes should be considered per-table.
each 'covering index' removes a physical table access from the plan & replaces it with index-only access.
investigate the plan costs & experiment with which tables are most worthwhile to replace by a covering index.
by this means, the multiplicative cost of large join plans can be significantly reduced.
For example:
select oi.title, c.name, c.address
from porderitem poi
join porder po on po.id = poi.fk_order
join customer c on c.id = po.fk_customer
where po.orderdate > ? and po.status = 'SHIPPING';
create index porder_custitem on porder (orderdate, id, status, fk_customer);
See:
http://literatejava.com/sql/covering-indexes-query-optimization/
Lets say you have a simple table with the below columns, you have only indexed Id here:
Id (Int), Telephone_Number (Int), Name (VARCHAR), Address (VARCHAR)
Imagine you have to run the below query and check whether its using index, and whether performing efficiently without I/O calls or not. Remember, you have only created an index on Id.
SELECT Id FROM mytable WHERE Telephone_Number = '55442233';
When you check for performance on this query you will be dissappointed, since Telephone_Number is not indexed this needs to fetch rows from table using I/O calls. So, this is not a covering indexed since there is some column in query which is not indexed, which leads to frequent I/O calls.
To make it a covered index you need to create a composite index on (Id, Telephone_Number).
For more details, please refer to this blog:
https://www.percona.com/blog/2006/11/23/covering-index-and-prefix-indexes/