On my ADW instance, indexes created by auto indexing are compressed by default but manually created indexes are not.
Why are manually created indexes not compressed by default?
Test case:
==========
SQL> create table a (a number);
Table created.
SQL> create index a_ind on a (a);
Index created.
SQL> select INDEX_NAME,COMPRESSION from dba_indexes where index_name='A_IND';
INDEX_NAME
--------------------------------------------------------------------------------
COMPRESSION
-------------
A_IND
DISABLED
SQL> drop index a_ind ;
Index dropped.
SQL> create index a_ind on a (a) compress;
Index created.
SQL> select INDEX_NAME,COMPRESSION from dba_indexes where index_name='A_IND';
INDEX_NAME
--------------------------------------------------------------------------------
COMPRESSION
-------------
A_IND
ENABLED
Automatic indexing has a preference for index compression as documented here:
https://docs.oracle.com/en/cloud/paas/autonomous-database/adbsa/autonomous-auto-index.html#GUID-D51A4F85-3AE5-4B7D-9B27-32CEDE8DBD79
Note: When automatic indexing is enabled, index compression for auto
indexes is enabled by default.
As of today you need to specify compression for your indexes manually.
Related
While running our process random tables are getting created with name like this BIN$xGf2aDIQ07bgUxnq9QrvVw==$0. Not sure how are they getting generated by themselves. Any idea?
These are objects in recycle bin, e.g.
SQL> select * From tab;
TNAME TABTYPE CLUSTERID
------------------------------ ------- ----------
ADHERENT TABLE
ADMIN_REGISTRATION TABLE
AUDIT_TABLE TABLE
AUDIT_TBL1 TABLE
BASE TABLE
BIN$v05hK8D0Y6HgU8lkAQqVqw==$0 TABLE
BIN$v05rTFDqUODgU8lkAQo1og==$0 TABLE
BIN$v05rTFDrUODgU8lkAQo1og==$0 TABLE
BIN$v05rTFDwUODgU8lkAQo1og==$0 TABLE
BIN$v3OGPz/pGrngU8lkAQoglg==$0 TABLE
BIN$v3OGPz/qGrngU8lkAQoglg==$0 TABLE
<snip>
How do objects get there? When you drop them. If you don't need them (i.e. you won't be retrieving anything from recycle bin), simply purge it:
SQL> purge recyclebin;
Recyclebin purged.
SQL> select * From tab;
TNAME TABTYPE CLUSTERID
------------------------------ ------- ----------
ADHERENT TABLE
ADMIN_REGISTRATION TABLE
AUDIT_TABLE TABLE
AUDIT_TBL1 TABLE
BASE TABLE
BONUS TABLE
BOOK TABLE
<snip> --> see? no more BIN$...
These objects created when you drop a table.
Ex:
drop table xyz;
but if you drop table with purge, the recyclebin tables does not get created
drop table xyz purge;
to delete these recyclebin tables you can use
purge recyclebin;
you can also delete recyclebin table one by one
PURGE TABLE test;
or
PURGE TABLE BIN$v05rTFDrUODgU8lkAQo1og==$0;
In case of Locally managed SYSTEM tablespace, when objects are created in a user tablespace (which is also locally managed) initial extents are getting allocated only after data insertion.
Whereas in the same scenario where SYSTEM tablespace is dictionary managed, initial extent gets allocated whenever table is created.
create tablespace lmt datafile 'df.f' size 5M extent management local;
conn scott/tiger
create table sample (id nuber) tablespace lmt;
select * from user_tables where table_name = 'SAMPLE';
initial extent is null when SYSTEM tablespace is Locally managed
initial extent has value when SYSTEM tablespace is dictionary managed
After data insertion, initial extent is allocated in case (1)
Is this the expected behavior? because as per Oracle docs, 'Oracle allocates space for initial extent when you create the schema object'
I think you're quoting (or rather, slightly paraphrasing) this part of the documentation:
INITIAL
Specify the size of the first extent of the object. Oracle allocates space for this extent when you create the schema object. ...
That isn't the whole story though. Tables can be created with deferred segment creation:
deferred_segment_creation
Use this clause to determine when the database should create the segment(s) for this table:
SEGMENT CREATION DEFERRED: This clause defers creation of the table segment — as well as segments for any LOB columns of the table, any indexes created implicitly as part of table creation, and any indexes subsequently explicitly created on the table — until the first row of data is inserted into the table. ...
SEGMENT CREATION IMMEDIATE: The table segment is created as part of this CREATE TABLE statement.
It looks like what you are seeing is nothing to do with the SYSTEM tablespace being locally managed, as you suspected, and that is just a coincidence. The difference is the default for segment creation, which is controlled by an initialisation parameter, deferred_segment_creation. From what you've shown that is set to TRUE in the database that has the locally-managed SYSTEM tablespace, and FALSE in the one that has that dictionary-managed.
You can get consistent behaviour by overriding the default, either to defer creation:
create table sample (id number) segment creation deferred tablespace lmt;
Table SAMPLE created.
set null "(null)"
select initial_extent from user_tables where table_name = 'SAMPLE';
INITIAL_EXTENT
--------------
(null)
or to create immediately:
create table sample (id number) segment creation immediate tablespace lmt;
Table SAMPLE created.
select initial_extent from user_tables where table_name = 'SAMPLE';
INITIAL_EXTENT
--------------
65536
Or you could change the initialisation parameters to be the same, of course, but that's more work and might affect other code that assumes the current behaviour.
I am working on our database users privileges, and I am facing a confusing issue. I first created a user (user1) without any quota and grant him the role CONNECT which have the system privilege (Create Session). After that I grant him the (Create Table) privilege. The user try to create a table and he succeeded to create a table in his tablespace!! my questions are:
Based on my understanding, the user must have a quota to create any objects, how this user created the table?
Is there any privilege or roles that give the user a quota implicitly?
Regards,
A quota isn't being granted implicitly. But creating a table doesn't necessarily require any storage, and so doesn't necessarily require a quota:
create table t42 (id number);
Table T42 created.
select segment_type, bytes from user_segments where segment_name = 'T42';
no rows selected
Adding data to the table does require storage, and therefore a quota:
insert into t42 (id) values (1);
1 row inserted.
select segment_type, bytes from user_segments where segment_name = 'T42';
SEGMENT_TYPE BYTES
------------------ ----------
TABLE 65536
If the owner doesn't have a quota on the tablespace then they will get an error when they try to insert; which is the case when they do have a quota and try to exceed it too of course (though the error will be different).
This behaviour is due to deferred segment creation; the default behaviour is controlled by an initialisation parameter. You can override that during table creation with the segment creation clause.
drop table t42 purge;
Table T42 dropped.
create table t42 (id number) segment creation immediate;
Table T42 created.
select segment_type, bytes from user_segments where segment_name = 'T42';
SEGMENT_TYPE BYTES
------------------ ----------
TABLE 65536
Read more in the documentation.
Incidentally, if you create a table with deferred segment creation then dbms_metadata.get_ddl shows that; if you then insert a row to force a segment to be created, dbms_metadata.get_ddl changes to SEGMENT CREATION IMMEDIATE. Which might not be expected. Truncating the table with the DROP ALL STORAGE clause will remove the segments, and revert the DDL to SEGMENT CREATION DEFERRED. Just something I noticed in passing.
I have the following PL/SQL code:
DROP TABLE TAB_PARAM;
BEGIN
EXECUTE IMMEDIATE 'CREATE TABLE TAB_PARAM
(
TABLE_OWNER VARCHAR2(30) NOT NULL,
TABLE_NAME VARCHAR2(30) NOT NULL,
COLUMN_NAME VARCHAR2(30) NOT NULL,
PATTERN VARCHAR2(1024),
TYPE_METHODE VARCHAR2(30) NOT NULL,
SEPARATEUR VARCHAR2(20),
ID VARCHAR2(30),
CONSTRAINT PK_TAB_PARAM PRIMARY KEY (TABLE_OWNER,TABLE_NAME,COLUMN_NAME) USING INDEX TABLESPACE IND_PARC_256M NOLOGGING
)
TABLESPACE TAB_PARC_256M NOLOGGING NOCACHE NOMONITORING NOPARALLEL';
commit;
END;
/
I don't understand the part :
CONSTRAINT PK_TAB_PARAM PRIMARY KEY (TABLE_OWNER,TABLE_NAME,COLUMN_NAME) USING INDEX TABLESPACE IND_PARC_256M NOLOGGING
Nor the part:
TABLESPACE TAB_PARC_256M NOLOGGING NOCACHE NOMONITORING NOPARALLEL';
I know that it is setting the ID as primary key of TAB_PARAM but then I do not get the index part.
Can anyone help me understand this code please?
Sometimes you can create a table in a disposable way , for example you need to read data via sqlloader from file and insert in table , after that you will select all records from table . This operation doesnt need indexes and doesnt need primary key . Anyway it is always good to create primary key when you create a table and Oracle will create index for you . You can create more indexes whenever you want since the table was created . Now suppose you wanna update the table using where condition on a field who is not a primary key , it could be a better decision create an index on that field . In addition when you create indexes or table , you can associate tablespaces (created before) and could be good practice separate tablespaces for tables and indexes . All those operations are DDL Statements . In Oracle you can check datafiles , tables and indexes with those queries select * from dba_data_files; select * from dba_tables; select * from dba_indexes;
For primary key oracle implicitly creates unique index in table's tablespace.
This part USING INDEX TABLESPACE allow us to indicate tablespace for this implicitly index.
NOLOGGING - data is modified with minimal logging (to mark new extents invalid and to record dictionary changes).
NOCACHE - Oracle doesn't store blocks in the buffer cache.
NOPARALLEL - Parallel execution is not allowed on this table.(Default value)
NOMONITORING - Disable statistics collection. Now is deprecated. There is other mechanism to collect statistic
Edit. from oracle doc
Oracle Database uses an existing index if it contains a unique set of values before enforcing the primary key constraint. The existing
index can be defined as unique or nonunique. When a DML operation is
performed, the primary key constraint is enforced using this existing
index.
If there already index hitting the pk columns , oracle will used it, else then Oracle Database generates a unique index.
so you can create a primary key in oracle without an index if there was already index for that columns , specifying an index is for performance issue. The purpose of an index in a table is to 'read' the data faster.
From the oracle document
An index is a schema object that contains an entry for each value that
appears in the indexed column(s) of the table or cluster and provides
direct, fast access to rows.
As for the TABLESPACE, its where the database object exists, so when you create a table you specify in which tablespace you want it to exists. Read more here oracle document
As for NOLOGGING NOCACHE NOMONITORING, so the table not to be logged in redo logs or cached, also related to performance issue.
Objective
Verify if it is true that insert records without PK/index plus create thme later is faster than insert with PK/Index.
Note
The point here is not about indexing takes more time (it is obvious), but the total cost (Insert without index + create index) is higher than (Insert with index). Because I was taught to insert without index and create index later as it should be faster.
Environment
Windows 7 64 bit on DELL Latitude core i7 2.8GHz 8G memory & SSD HDD
Oracle 11G R2 64 bit
Background
I was taught that insert records without PK/Index and create them after insert would be faster than insert with PK/Index.
However 1 million record inserts with PK/Index was actually faster than creating PK/Index later, approx 4.5 seconds vs 6 seconds, with the experiments below. By increasing the records to 3 million (999000 -> 2999000), the result was the same.
Conditions
The table DDL is below. One bigfile table space for both data and
index.
(Tested a separate index tablespace with the same result & inferior overall perforemace)
Flush the buffer/spool before each run.
Run the experiment 3 times each and made sure the results
were similar.
SQL to flush:
ALTER SYSTEM CHECKPOINT;
ALTER SYSTEM FLUSH SHARED_POOL;
ALTER SYSTEM FLUSH BUFFER_CACHE;
Question
Would it be actually true that "insert witout PK/Index + PK/Index creation later" is faster than "insert with PK/Index"?
Did I make mistakes or missed some conditions in the experiment?
Insert records with PK/Index
TRUNCATE TABLE TBL2;
ALTER TABLE TBL2 DROP CONSTRAINT PK_TBL2_COL1 CASCADE;
ALTER TABLE TBL2 ADD CONSTRAINT PK_TBL2_COL1 PRIMARY KEY(COL1) ;
SET timing ON
INSERT INTO TBL2
SELECT i+j, rpad(TO_CHAR(i+j),100,'A')
FROM (
WITH DATA2(j) AS (
SELECT 0 j FROM DUAL
UNION ALL
SELECT j+1000 FROM DATA2 WHERE j < 999000
)
SELECT j FROM DATA2
),
(
WITH DATA1(i) AS (
SELECT 1 i FROM DUAL
UNION ALL
SELECT i+1 FROM DATA1 WHERE i < 1000
)
SELECT i FROM DATA1
);
commit;
1,000,000 rows inserted.
Elapsed: 00:00:04.328 <----- Insert records with PK/Index
Insert records without PK/Index and create them after
TRUNCATE TABLE TBL2;
ALTER TABLE &TBL_NAME DROP CONSTRAINT PK_TBL2_COL1 CASCADE;
SET TIMING ON
INSERT INTO TBL2
SELECT i+j, rpad(TO_CHAR(i+j),100,'A')
FROM (
WITH DATA2(j) AS (
SELECT 0 j FROM DUAL
UNION ALL
SELECT j+1000 FROM DATA2 WHERE j < 999000
)
SELECT j FROM DATA2
),
(
WITH DATA1(i) AS (
SELECT 1 i FROM DUAL
UNION ALL
SELECT i+1 FROM DATA1 WHERE i < 1000
)
SELECT i FROM DATA1
);
commit;
ALTER TABLE TBL2 ADD CONSTRAINT PK_TBL2_COL1 PRIMARY KEY(COL1) ;
1,000,000 rows inserted.
Elapsed: 00:00:03.454 <---- Insert without PK/Index
table TBL2 altered.
Elapsed: 00:00:02.544 <---- Create PK/Index
Table DDL
CREATE TABLE TBL2 (
"COL1" NUMBER,
"COL2" VARCHAR2(100 BYTE),
CONSTRAINT "PK_TBL2_COL1" PRIMARY KEY ("COL1")
) TABLESPACE "TBS_BIG" ;
The current test case is probably good enough for you to overrule the "best practices". There are too many variables involved to make a blanket statement that "it's always best to leave the indexes enabled". But you're probably close enough to say it's true for your environment.
Below are some considerations for the test case. I've made this a community wiki in the hopes that others will add to the list.
Direct-path inserts. Direct-path writes use different mechanisms and may work completely differently. Direct-path inserts can often be significantly faster than regular inserts, although they have some complicated restrictions (for example, triggers must be disabled) and disadvantages (the data is not immediately backed-up). One particular way it affects this scenario is that NOLOGGING for indexes only applies during index creation. So even if a direct-path insert is used, an enabled index will always generate REDO and UNDO.
Parallelism. Large insert statements often benefit from parallel DML. Usually it's not worth worrying about the performance of bulk loads until it takes more than several seconds, which is when parallelism starts to be useful.
Bitmap indexes are not meant for large DML. Inserts or updates to a table with a bitmap index can lock the whole table and lead to disastrous performance. It might be helpful to limit the test case to b-tree indexes.
Add alter system switch logfile;? Log file switches can sometimes cause performance issues. The tests would be somewhat more consistent if they all started with empty logfiles.
Move data generation logic into a separate step. Hierarchical queries are useful for generating data but they can have their own performance issues. It might be better to create in intermediate table to hold the results, and then only test inserting the intermediate table into the final table.
It's true that it is faster to modify a table if you do not also have to modify one or more indexes and possibly perform constraint checking as well, but it is also largely irrelevant if you then have to add those indexes. You have to consider the complete change to the system that you wish to effect, not just a single part of it.
Obviously if you are adding a single row into a table that already contains millions of rows then it would be foolish to drop and rebuild indexes.
However, even if you have a completely empty table into which you are going to add several million rows it can still be slower to defer the indexing until afterwards.
The reason for this is that such an insert is best performed with the direct path mechanism, and when you use direct path inserts into a table with indexes on it, temporary segments are built that contain the data required to build the indexes (data plus rowids). If those temporary segments are much smaller than the table you have just loaded then they will also be faster to scan and to build the indexes from.
the alternative, if you have five index on the table, is to incur five full table scans after you have loaded it in order to build the indexes.
Obviously there are huge grey areas involved here, but well done for:
Questioning authority and general rules of thumb, and
Running actual tests to determine the facts in your own case.
Edit:
Further considerations -- you run a backup while the indexes are dropped. Now, following an emergency restore, you have to have a script that verifies that all indexes are in place, when you have the business breathing down your neck to get the system back up.
Also, if you absolutely were determined to not maintain indexes during a bulk load, do not drop the indexes -- disable them instead. This preserves the metadata for the indexes existence and definition, and allows a more simple rebuild process. Just be careful that you do not accidentally re-enable indexes by truncating the table, as this will render disabled indexes enabled again.
Oracle has to do more work while inserting data into table having an index. In general, inserting without index is faster than inserting with index.
Think in this way,
Inserting rows in a regular heap-organized table with no particular row order is simple. Find a table block with enough free space, put the rows randomly.
But, when there are indexes on the table, there is much more work to do. Adding new entry for the index is not that simple. It has to traverse the index blocks to find the specific leaf node as the new entry cannot be made into any block. Once the correct leaf node is found, it checks for enough free space and then makes the new entry. If there is not enough space, then it has to split the node and distribute the new entry into old and new node. So, all this work is an overhead and consumes more time overall.
Let's see a small example,
Database version :
SQL> SELECT banner FROM v$version where ROWNUM =1;
BANNER
--------------------------------------------------------------------------------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
OS : Windows 7, 8GB RAM
With Index
SQL> CREATE TABLE t(A NUMBER, CONSTRAINT PK_a PRIMARY KEY (A));
Table created.
SQL> SET timing ON
SQL> INSERT INTO t SELECT LEVEL FROM dual CONNECT BY LEVEL <=1000000;
1000000 rows created.
Elapsed: 00:00:02.26
So, it took 00:00:02.26. Index details:
SQL> column index_name format a10
SQL> column table_name format a10
SQL> column uniqueness format a10
SQL> SELECT index_name, table_name, uniqueness FROM user_indexes WHERE table_name = 'T';
INDEX_NAME TABLE_NAME UNIQUENESS
---------- ---------- ----------
PK_A T UNIQUE
Without Index
SQL> DROP TABLE t PURGE;
Table dropped.
SQL> CREATE TABLE t(A NUMBER);
Table created.
SQL> SET timing ON
SQL> INSERT INTO t SELECT LEVEL FROM dual CONNECT BY LEVEL <=1000000;
1000000 rows created.
Elapsed: 00:00:00.60
So, it took only 00:00:00.60 which is faster compared to 00:00:02.26.