Differences between Unique and Non-Unique Indexes (Part I) December 18, 2007 Posted by Richard Foote in Constraints, Deferrable Constraints, Index Internals, Indexing Tricks, Novalidate Constraints, Oracle Indexes, Primary Key, Unique Indexes. Primary key with Non Unique Index Hi Team,I was just playing around with some stuff so thought sharing with you.My understanding for Primary key was that Oracle uses Unique Index to enforce primary key constraint on table's column to be unique.Below is the code that I tried, in which I created Non-Unique Index on Primary
FETCH #2:c=0,e=23057,p=2,cr=2,cu=0,mis=0,r=1,dep=0,og=1,tim=24452838728 Check out Chris Saxon's full fundamentals class. Is there any way to do it with out dropping and recreating as a unique constraint? If we now use a column that has on average 4 occurrences for each distinct column value (with 250,000 distinct values in a 1 million row table), the differences between a Bitmap and a B-Tree index begin to widen significantly. Fortified Bicycle appears to gone out of business -- where to get one of their replacement bike light batteries? kdxcofbo 38=0x26 You can create function-based indexes. But there are some performance considerations: a unique index would be smaller and faster. User reports a bug, send it to QA first or Programmer first?
I’ve previously discussed the differences between Unique and Non-Unique Indexes and my general preference for using Unique Indexes where possible and appropriate: https://richardfoote.wordpress.com/2007/12/18/differences-between-unique-and-non-unique-indexes-part-i/, https://richardfoote.wordpress.com/2007/12/21/differences-between-unique-and-non-unique-indexes-part-ii/, https://richardfoote.wordpress.com/2007/12/30/differences-between-unique-and-non-unique-indexes-part-iii/.
It is clear in this table that the B-tree index is smaller than the bitmap index on the EMPNO culumn. header address 425713756=0x195fe05c How can that possibly be more efficient than reading directly from the only table block in the table segment that contains rows ? Is it still necessary to use packages T1 and utf8 in editor TeXstudio? Why can so little digital information be stored on a cassette tape? More specifically, if you have a table with 50 columns and users frequently query on 10 of themeither the combination of all 10 columns or sometimes a single culumncreating a B-tree index will be very difficult. Total for a B-Tree Index would be: In the above table, culumn EMPNO has 100-percent distinct values and is a good candidate to become a primary key.
row#0[8024] flag: ——, lock: 0, len=12 Because data is frequently updated in and deleted from ulTP applications, bitmap indexes can cause a serious locking problem in these situations. https://docs.oracle.com/database/121/SQLRF/statements_7002.htm#i2153235, Oracle documentation (9i): Index Organized Tables. Q&A with the creators of Next.js on version 9.5, Goodbye, Prettify.
Details in Oracle documentation (9i): Index Organized Tables: An index-organized table - in contrast to an ordinary table - has its own way of structuring, storing, and indexing data. Now in our specific example, that’s a saving of 1 consistent get and effectively 2 latch gets by using the Non-Unique index vs. performing the FTS on this effectively 1 block table. SQL> ALTER TABLE ziggy ADD CONSTRAINT ziggy_pk PRIMARY KEY(id); OK, let’s see the current number of physical writes direct operations: SQL> INSERT /*+ APPEND */ INTO ziggy SELECT 10002, ‘NEW’ FROM dual; And the number of physical writes direct has now increased.
Primary key with Non Unique Index Hi Team,I was just playing around with some stuff so thought sharing with you.My understanding for Primary key was that Oracle uses Unique Index to enforce primary key constraint on table's column to be unique.Below is the code that I tried, in which I created Non-Unique Index on Primary We suggest you try the following to help find what you’re looking for: Understanding the proper application of each index can have a big impact on performance. Create a bitmap index on the SAL culumn of the TEST_NORMAL table. WAIT #2: nam=’SQL*Net message to client’ ela= 5 driver id=1111838976 #bytes=1 p3=0 obj#=12446 tim=24452815631 To learn more, see our tips on writing great answers. KDXCOLEV Flags = – – – Now for the tests; equality predicates first: When the queries were executed for different set of values, the resulting output, as shown in the tables below, reveals that the numbers of consistent gets and physical reads are identical. WAIT #2: nam=’SQL*Net message to client’ ela= 3 driver id=1111838976 #bytes=1 p3=0 obj#=99586 tim=24452839399. col 1; len 6; (6): 01 c1 e7 8a 00 01 There is no possibility for now to create a clustered index, but it it possible to add some features to achieve nearly same result: (B-Tree & Bitmap INDX)
Oracle - Materialized View alter structure so slow, Oracle Constraints: mix unique and check constraints, Cannot trouble dropping unique constraint.
The answer will perhaps surprise many, especially those that only consider Bitmap Indexes to be viable for so-called “low cardinality”columns on large tables where there could be many millions of occurrences of each distinct value in a column.
What about the HWM ?
The following “db file sequential read” wait event on obj#=99586 corresponds to the table segment and file#=7 blockblock#=117898 corresponds to the table block containing the rows. The number of such operations has not changed. What if the data is poorly clustered: OK, the Bitmap index is now larger by 178 leaf blocks than it was before, but the equivalent B-Tree index is still larger by 10.6%. Stack Overflow for Teams is a private, secure spot for you and WHERE s.statistic# =n.statistic# AND s.sid = 134 AND n.name LIKE ‘consistent%’; And again after the SELECT on the small table: The consistent gets indeed increased by 3 rather than 4 but again as in the FTS example, none of these consistent gets were the cheaper “consistent gets – examination” (to be discussed later). We note that Oracle has indeed reused the previously deleted index entry and has simply updated the rowid with the new rowid value. Once this location is established, the row never completely moves. Yep, that’s gone up by one as well. kdxcofeo 8025=0x1f59
In the first example, 1/3 of all values are unique while the remaining 2/3 of values have just 2 occurrences of each value.
Note also (and this is the critical bit for Unique indexes), because the actual index value remains the same, the order of the index entry within the index must also remain the same. Is it bad to look at your hands while playing piano? a. Consequently, you can conclude that for a normal-cardinality culumn, the optimizer decisions for the two types of indexes were the same and there were no significant differences between the I/Os. From the TEST_NORMAL table, you need the employee number of all the male employees whose monthly salaries equal any of the fullowing values: This is a typical data warehouse query, which, of course, you should never execute on an ulTP system. SQL> INSERT /*+ APPEND */ INTO ziggy SELECT 10003, ‘NEW’ FROM dual; Yes indeed, Direct-Path Inserts are definitely now occurring now we have a Unique Index policing our PK constraint.
Create a bitmap index on this culumn, and then execute some queries with equality predicates. ; INDEX_NAME, INDEX_TYPE, TABLE_NAME, TABLE_TYPE, CLUSTERING_FACTOR kdxlende 0 Create a bitmap index on the SAL culumn, and then execute some queries with equality predicates and some with range predicates. 4 bytes index column How does the highlight.js change affect Stack Overflow specifically?
b. OQ: What is the difference between unique index and non unique index in Oracle?
These examples will help DBAs understand that the usage of bitmap indexes is not in fact cardinality dependent but rather application dependent. Here’s an actual, “real life” example. Therefore, if we were to delete and re-insert the same index value within a single transaction, Oracle does not need to create a new index entry and can simply reuse the existing, deleted index entry. In the little example we went through, Oracle still needs to access 2 distinct table blocks and perform 4 consistent get operations to read this one row of interest. The b-trees are absolutely identical, and the SQL will use the index in the same fashion.
KDXCOLEV Flags = – – – Play the long game when learning to code. There are several disadvantages to using a bitmap index on a unique culumn--one being the need for sufficient space (and Oracle does not recommend it). NO !! WAIT #2: nam=’db file sequential read’ ela= 5592 file#=7 block#=117898 blocks=1 obj#=99586 tim=24452838643 kdxcolok 0 bytes for the actual bitmap sequence As such, Oracle doesn’t permit the use of a Bitmap Index on a declared unique column or to police a unique constraint. Still I have doubt. Notice how the rowid is an additional index column within the index entry for the Non-Unique index. kdxcosdc 0
Now let's execute the queries with equality predicates for different sets of values: Now we will drop this bitmap index and create a B-tree index on the EMPNO culumn. kdxleprv 0=0x0 The clustering factor of the B-tree index is much nearer to the number of blocks in a table; for that reason, the B-tree index is efficient for range predicate queries.
Making statements based on opinion; back them up with references or personal experience.
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