I would like to spare some tables in my database.
One table for example has a simple Primary-Key-ID column and a VARCHAR2 column.
The VARCHAR2 column has NO duplicate values, yet different unique IDs.
The PK column of this table is just referenced once as a foreign key in another table.
My thoughts are now to insert the values from the VARCHAR2 column into the the table which has held the primary key.
I could now remove the foreign key reference, delete the table and gain a new column with all the (duplicate) VARCHAR2 values. These I would like to compress in a unique/distinct way.
I have heard about index in the Oracle Database to compress column(s) but I am not quite sure which index I need or how to use them...
The underlying feature (and storage savings) should be about as the same as it was with the previous table of unique values and the foreign key reference.
Thank you for your help in advance!
Oracle basic compression allows us to compress tables. It comes with several distinct limitations, not the least of which is that it isn't suitable for OLTP databases. Direct path inserts, updates and deletes don't benefit. So you can't do what you want that way. If your organisation has sprung for the Advanced Compression licence then you have more options, but the compression still works on the table not an individual column.
I think you've confused things with index compression, which does operate on columns, as it allows us to compress the leading column(s) of a compound index. But it's worth applying only when there's a lot of repetition in those columns. If your index has a unique ID for the leading column than compression will actually increase the total amount of space taken. (Just one reason why compound indexes should be built with the least selective column first and the most selective column last.)
Your table is a classic key-value lookup table. So you could consider converting it into an index-organized table. You would save yourself a bit of space by maintaining only a specialized index instead of a table and its primary key index. Find out more
Related
We have a table that doesn't have much data. The table has 3 partitions and we are deleting data in one partition only.
delete from table AB partition(A) where id=value;
here id has an index also but still delete is slow.
The datatype of id is varchar2 and the value is number.
Please help me to understand why the delete statement is slow.
I don't think the index has much use in this case. It has to evaluate every single row in the partition to see if it matches id=value. Typically this will be a full table scan and no index will be used. It totally depends on the number of rows in the partition how long it will take. But maybe i did not understand the question properly. I presumed "value" is a column in the same table, like ID.
I have a huge table with one int PRIMARY KEY IDENTITY column.
I guess making the SELECT query using that primary key is the fastest way for the database to find the row in the table isn't it?
If that is true i still have a question.
Is that query as fast as a call to a dictionary by key or the database still has to read all the rows from the beginning (the Primary Key column) till it finds the row itself?
Thanks in advance ^^
Using primary key is obviously the fastest way to access a particular row.
If you want to understand how it works, you have to understand how index works.
In general it works like that :
Let's say you have a table t1(col1,col2...col10) and you have an index on col1.
Index on col1 means that you have some data structure which contains pairs (col1, rec_id)
and rec_id allows direct access to row with appropriate col1.
The data structure is ordered by col1 and therefore allows efficient searching by col1.
I think searching in dictionary works per dictionary search algorithm which should be more like binary search kind.
When you declare a column as Primary key in table, then that column is indexed, hence it should be working based on hashing principle, so searching is definitely NOT row by row as you mentioned.
Finally, yes it is the common and fast way, but you should be selective about the number of columns and rows you need in your sql query. Avoid fetching large number of rows per select call.
I have a table in Oracle Database which has 60 columns. Following is the table structure.
ID NAME TIMESTAMP PROERTY1 ...... PROPERTY60
This table will have many rows. the size of the table will be in GBs. But the problem with the table structure is that in future if I have to add a new property, I have to change the schema. To avoid that I want to change the table structure to following.
ID NAME TIMESTAMP PROPERTYNAME PROPERTYVALUE
A sample row will be.
1 xyz 40560 PROPERTY1 34500
In this way I will be able to solve the issue but the size of the table will grow bigger. Will it have any impact on performance in terms on fetching data. I am new to Oracle. I need your suggestion on this.
if I have to add a new property, I have to change the schema
Is that actually a problem? Adding a column has gotten cheaper and more convenient in newer versions of Oracle.
But if you still need to make your system dynamic, in a sense that you don't have to execute DDL for new properties, the following simple EAV implementation would probably be a good start:
CREATE TABLE FOO (
FOO_ID INT PRIMARY KEY
-- Other fields...
);
CREATE TABLE FOO_PROPERTY (
FOO_ID INT REFERENCES FOO (FOO_ID),
NAME VARCHAR(50),
VALUE VARCHAR(50) NOT NULL,
CONSTRAINT FOO_PROPERTY_PK PRIMARY KEY (FOO_ID, NAME)
) ORGANIZATION INDEX;
Note ORGANIZATION INDEX: the whole table is just one big B-Tree, there is no table heap at all. Properties that belong to the same FOO_ID are stored physically close together, so retrieving all properties of the known FOO_ID will be cheap (but not as cheap as when all the properties were in the same row).
You might also want to consider whether it would be appropriate to:
Add more indexes in FOO_PROPERTY (e.g. for searching on property name or value). Just beware of the extra cost of secondary indexes in index-organized tables.
Switch the order of columns in the FOO_PROPERTY PK - if you predominantly search on property names and rarely retrieve all the properties of the given FOO_ID. This would also make the index compression feasible, since the leading edge of the index is now relatively wide string (as opposed to narrow integer).
Use a different type for VALUE (e.g. RAW, or even in-line BLOB/CLOB, which can have performance implications, but might also provide additional flexibility). Alternatively, you might even have a separate table for each possible value type, instead of stuffing everything in a string.
Separate property "declaration" to its own table. This table would have two keys: beside string NAME it would also have integer PROPERTY_ID which can then be used as a FK in FOO_PROPERTY instead of the NAME (saving some storage, at the price of more JOIN-ing).
I have a table Customer_Chronics in Oracle 11g.
The table has three key columns as shown below :
branch_code
customer_id
period
I have partitioned by table by list of branch_code, and now I'm having dilemma. Which is better:
Create unique index indexNumberOne on Customer_Chronics (PERIOD, CUSTOMER_ID);
Create unique index indexNumberTwo on Customer_Chronics (branch_code, PERIOD, CUSTOMER_ID);
The actual data must be unique by period, customer_id. If I put a unique index only on these two columns Oracle will check all partitions on the table when inserting new records?
The only way to enforce uniqueness is with a unique constraint on the columns of interest. So that's your first option. The database will check all values across all partitions it this case. But as it's a unique index that shouldn't take too long no matter how big the table gets (if that's your concern).
Yes, If you put unique index on that two columns only, Oracle will create a global index and will check all partitions. This is one of challenges I face sometime because we prefer local indexs for big tables (small tables should be OK).
what is use-case of IOT (Index Organized Table) ?
Let say I have table like
id
Name
surname
i know the IOT but bit confuse about the use case of IOT
Your three columns don't make a good use case.
IOT are most useful when you often access many consecutive rows from a table. Then you define a primary key such that the required order is represented.
A good example could be time series data such as historical stock prices. In order to draw a chart of the stock price of a share, many rows are read with consecutive dates.
So the primary key would be stock ticker (or security ID) and the date. The additional columns could be the last price and the volume.
A regular table - even with an index on ticker and date - would be much slower because the actual rows would be distributed over the whole disk. This is because you cannot influence the order of the rows and because data is inserted day by day (and not ticker by ticker).
In an index-organized table, the data for the same ticker ends up on a few disk pages, and the required disk pages can be easily found.
Setup of the table:
CREATE TABLE MARKET_DATA
(
TICKER VARCHAR2(20 BYTE) NOT NULL ENABLE,
P_DATE DATE NOT NULL ENABLE,
LAST_PRICE NUMBER,
VOLUME NUMBER,
CONSTRAINT MARKET_DATA_PK PRIMARY KEY (TICKER, P_DATE) ENABLE
)
ORGANIZATION INDEX;
Typical query:
SELECT TICKER, P_DATE, LAST_PRICE, VOLUME
FROM MARKET_DATA
WHERE TICKER = 'MSFT'
AND P_DATE BETWEEN SYSDATE - 1825 AND SYSDATE
ORDER BY P_DATE;
Think of index organized tables as indexes. We all know the point of an index: to improve access speeds to particular rows of data. This is a performance optimisation of trick of building compound indexes on sub-sets of columns which can be used to satisfy commonly-run queries. If an index can completely satisy the columns in a query's projection the optimizer knows it doesn't have to read from the table at all.
IOTs are just this approach taken to its logical confusion: buidl the index and throw away the underlying table.
There are two criteria for deciding whether to implement a table as an IOT:
It should consists of a primary key (one or more columns) and at most one other column. (okay, perhaps two other columns at a stretch, but it's an warning flag).
The only access route for the table is the primary key (or its leading columns).
That second point is the one which catches most people out, and is the main reason why the use cases for IOT are pretty rare. Oracle don't recommend building other indexes on an IOT, so that means any access which doesn't drive from the primary key will be a Full Table Scan. That might not matter if the table is small and we don't need to access it through some other path very often, but it's a killer for most application tables.
It is also likely that a candidate table will have a relatively small number of rows, and is likely to be fairly static. But this is not a hard'n'fast rule; certainly a huge, volatile table which matched the two criteria listed above could still be considered for implementations as an IOT.
So what makes a good candidate dor index organization? Reference data. Most code lookup tables are like something this:
code number not null primary key
description not null varchar2(30)
Almost always we're only interested in getting the description for a given code. So building it as an IOT will save space and reduce the access time to get the description.