The execution time of stored procedures were taking so much of time.so we were asked to rebuild indexes for the entire schema and compute statistics.
Here my question is compute statistics has to be done on the table or index?
this is what i tried
ALTER INDEX SHIP_UNIT_PK REBUILD;
exec dbms_stats.gather_index_stats('stage','SHIP_UNIT_PK ');
PL/SQL procedure successfully completed .
What has to be done/or checked after this?
The only thing to do is, I think:
analyze table <yourtable> compute statistics;
But anyway if your table was not changed that much, it is enough to rebuild index.
GATHER_TABLE_STATS Procedure
This procedure gathers table and column (and index) statistics
Related
I needed to truncate and reload a table.
I learned that truncate needs stats gathering on the table as its successor process so the database gets the actual statistics, otherwise previous stats are not cleared by the truncate statement.
After doing these two operations (truncate and stats gathering on the empty table), ran the insert... but don't see new statistics in all_tab_statistics table for my table. Sample_size is still 0.
Why is that? Shouldn't have Oracle done the automatic stats gathering after the insert?
Do I need to rerun the stats or is it just fine considering the performance around this table (please note it's going to truncate and reload each time)?
Consider the following approach. It has the advantage of the table always being present.
Create an empty new table like the old one.
Load the data into the new table. This is the slowest step.
Do whatever cleanup you might need, such as refreshing the statistics.
RENAME tables to swap the new table in place. This step is fast enough so you won't notice.
I know it's a long time since I posted my question above. But recently, we again faced the similar situation and this time below steps worked towards a much better performance on a table with 800 million rows.
Take a backup of the original table.
Truncate the original table.
Gather stats on the truncated table, so that statistics show 0 in the DB. Us CASCADE=>TRUE in the command to also include indexes in the process.
Drop the indexes on the truncated table and Insert the required data from the backup table.
Recreate the indexes and gather stats again (ofcourse, with CASCADE=>TRUE; however recreation of the indexes should ideally have calculated the appropriate stats).
Drop the backup table if not needed.
We are running Oracle 11g and have some partitioned tables. I am trying to write an automated process to script out the indexes on these tables. (Basically when we do bulk loads, we want to drop all the indexes beforehand and recreate them afterward.)
The problem I have is knowing how to script out the partitioned indexes. Some are created with "LOCAL STORE IN (tablespacename)" and others just with "LOCAL" (which stores index extents in the same partition as the data). In either case, dba_indexes.tablespace_name is null, and I have having a heck of a time scripting out the two different cases correctly.
I know I can simply re-run the original DDL to recreate the indexes, but multiple parts of the organization can make changes, and there would be less risk if the loader tool could be self-contained and simply rebuild whatever was there to begin with.
I can query dba_ind_subpartitions, and if the tablespace_name values for every subpartition all match, then I can assume/infer that I should STORE IN that tablespace name. But, if the table is in a small single-partition state (e.g. newly created or just after archival), then the ones created with just LOCAL also match this test, so this is also not a perfect way of telling them apart.
I can compare the names of the index subpartition tablespaces to the data table partition tablespaces, and if they match, then I can assume/infer that those should be created with just LOCAL. But, that drags a bunch of extra tables into my query and makes it really hard to read, so I am worried about maintainability going forward. Plus, it just seems like a kludge.
It seems like there should be someplace in Oracle's data dictionaries where it is simply keeping track of this, and where I can just directly look it up instead of having to do a bunch of math and rely on assumptions. But, I have done a good deal of digging and haven't yet found it. So, any help would be much appreciated.
Although an insert alone is faster without the presence of indexes, have you benchmarked a load into tables with indexes enabled and established that it is slower than disabling (more robust than dropping!) and rebuilding them?
When you direct path insert into a table with indexes, Oracle optimises the index maintenance process by creating temporary segments to hold just the data required for the index builds. This generally allows the index maintenance to scan much smaller segments than otherwise required -- the temp segments plus the existing indexes.
Well, as jonearles describes, the dbms_metadata package is the way to generate DDL for existing objects.
But, it seems to me, this is more work than is required for what you're trying to achieve. If this is all for loading data, I recommend you simply alter the indexes to be unusable, set 'skip_unusable_indexes=true', do the data load, and the rebuild the indexes.
This should achieve what you want, without having to drop and re-create the indexes.
DBMS_METADATA.GET_DDL is easier than querying the data dictionary:
--Sample table and index.
create table test1(a number);
create index test1_idx on test1(a);
--Store the DDL, drop the index, then re-create it.
declare
ddl_before clob;
begin
ddl_before := dbms_metadata.get_ddl('INDEX', 'TEST1_IDX');
execute immediate 'drop index test1_idx';
--Do some processing here.
execute immediate ddl_before;
end;
/
I am trying to update statistics to some of our tables whose names I receive as input to my procedure. But, I couldn't compile the procedure with the below code.
update index statistics #tableName
Aren't dynamic table names allowed? Or, would the below statement work?
select #statsCmd = 'update index statistics '+#tableName
exec(#statsCmd)
Also, what are the notable differences between "update statistics" and "update index statistics"?
It does appear that update statistics does not allow dynamic table names, but the second statement should work without issue.
Regarding update statistics & update index statistics:
Update statistics can be run against tables without indexes, and other non-index objects, as well as against indexes. If run against an index, it actually executes an update index statistics behind the scenes. Update index statistics only updates statistics for the indices on the specified table.
Also, have you looked into using the Job Scheduler, and the datachange function to automate your update statistics?
Is this a misconception?
Sybase query optimizer can not consider index on temporary table if it is created and used in same batch or procedure.
I'm a bit confused about this as there seem to be differing opinion.
Thanks heaps.
That's quite simple.
The optimizer calculates the query plan before execution and the temporary table is created when executing the stored procedure.
A way to pass this problem is to create the temporary table and its index before the execution of the stored procedure :)
I am writing a data conversion in PL/SQL that processes data and loads it into a table. According to the PL/SQL Profiler, one of the slowest parts of the conversion is the actual insert into the target table. The table has a single index.
To prepare the data for load, I populate a variable using the rowtype of the table, then insert it into the table like this:
insert into mytable values r_myRow;
It seems that I could gain performance by doing the following:
Turn logging off during the insert
Insert multiple records at once
Are these methods advisable? If so, what is the syntax?
It's much better to insert a few hundred rows at a time, using PL/SQL tables and FORALL to bind into insert statement. For details on this see here.
Also be careful with how you construct the PL/SQL tables. If at all possible, prefer to instead do all your transforms directly in SQL using "INSERT INTO t1 SELECT ..." as doing row-by-row operations in PL/SQL will still be slower than SQL.
In either case, you can also use direct-path inserts by using INSERT /*+APPEND*/, which basically bypasses the DB cache and directly allocates and writes new blocks to data files. This can also reduce the amount of logging, depending on how you use it. This also has some implications, so please read the fine manual first.
Finally, if you are truncating and rebuilding the table it may be worthwhile to first drop (or mark unusable) and later rebuild indexes.
Regular insert statements are the slowest way to get data in a table and not meant for bulk inserts. The following article references a lot of different techniques for improving performance: http://www.dba-oracle.com/oracle_tips_data_load.htm
Drop the index, then insert the rows, then re-create the index.
If dropping the index doesn't speed things up enough, you need the Oracle SQL*Loader:
http://www.oracle.com/technology/products/database/utilities/htdocs/sql_loader_overview.html
Suppose you have taken eid,ename,sal,job. So create a table first as:
SQL>create table tablename(eid number, ename varchar2(20),sal number,job char(10));
Now insert data:-
SQL>insert into tablename values(&eid,'&ename',&sal,'&job');
Check this link
http://www.dba-oracle.com/t_optimize_insert_sql_performance.htm
main points to consider for your
case is to use Append hint as this
will directly append into the table
instead of using freelist. If you can afford to turn off logging than use append with nologging hint to do it
Use a bulk insert instead instead of iterating in PL/SQL
Use sqlloaded to load the data directly into the table if you are getting data from a file feed
Here are my recommendations on fast insert.
Trigger - Disable any triggers associated with a table. Enable after Inserts are complete.
Index - Drop Index and re-create it after your Inserts are complete.
Stale stats - Re-analyze table and index stats.
Index de-fragmentation - Rebuild Index if needed
Use No Logging -Insert using INSERT APPEND (Oracle only). This approach is very risky approach, no redo logs are generated therefore you can’t do a rollback - make a backup of table before you start and don't try on live tables. Check if your db has similar option
Parallel Insert: Running parallel insert will get the job faster.
Use Bulk Insert
Constraints - Not much overhead during inserts but still a good idea to check, if it is still slow after even after step 1
You can learn more on http://www.dbarepublic.com/2014/04/slow-insert.html
Maybe one of your best option is to avoid Oracle as much as possible actually.
I've been baffled by this myself, but very often a Java process can outperform many of the Oracle's utilities which either use OCI (read: SQL Plus) or will take up so much of your time to get right (read: SQL*Loader).
This doesn't prevent you to use specific hints either (like /APPEND/).
I've been pleasantly surprised each time I've turned to that kind of solution.
Cheers,
Rollo