Oracle 10g temp tables

Question

I'm trying to convert the permanent tables used in a stored procedure to global temp tables. I've looked at the stats on these permanent tables and some have tens of millions of rows of data and are on the order if gigabytes in size (up to 10 GB).

So,

CREATE TABLE my_table (  
  column1 NUMBER,  
  column2 NUMBER,  
  etc...  
)  
TABLESPACE BIGTABLESPACE  
NOLOGGING  
NOCOMPRESS  
NOCACHE  
NOPARALLEL  
MONITORING;  

should become

CREATE GLOBAL TEMPORARY TABLE my_table (  
  column1 NUMBER,  
  column2 NUMBER,  
  etc..  
)  
ON COMMIT PRESERVE ROWS;  

I'm creating an equivalent global temporary table with rows that should be preserved until the end of the session for each existing permanent table. This global temp table will be used in the procedure instead of the permanent table.
(EXECUTE IMMEDIATE 'TRUNCATE ...' at the start, and INSERT /*+ APPEND */ INTO at some later point)

All of the permanent tables have been created in a big tablespace BIGTABLESPACE

The Oracle docs state that the global temporary table will be created in the user's temp tablespace (I assume this is TEMP). The problem with this is that the TEMP tablespace is small and the extents are not set to grow to the size I need them to grow during the procedure.

The TEMP tablespace was created during the database creation

create database "$oracle\_sid"  
   user sys identified by "$sys\_password"  
   user system identified by "$system\_password"  
   set default bigfile tablespace  
   controlfile   reuse  
   maxdatafiles  256  
   maxinstances  $maxinstances  
   maxlogfiles   16  
   maxlogmembers 3  
   maxloghistory 1600  
   noarchivelog  
   character set WE8MSWIN1252  
   national character set AL16UTF16  
   datafile  
      '$oracle\_home/oradata/$oracle\_sid/system01.dbf' size 512M  
   logfile  
      '$oracle\_home/oradata/$oracle\_sid/redo01.log' size 1G,  
      '$oracle\_home/oradata/$oracle\_sid/redo02.log' size 1G,  
      '$oracle\_home/oradata/$oracle\_sid/redo03.log' size 1G  
   sysaux datafile  
      '$oracle\_home/oradata/$oracle\_sid/sysaux01.dbf' size 256M  
   default temporary tablespace temp tempfile  
      '$oracle\_home/oradata/$oracle\_sid/temp01.dbf' size 5G  
   undo tablespace "UNDOTBS1" datafile  
      '$oracle\_home/oradata/$oracle\_sid/undotbs01.dbf' size 5G;  

The permanent tables (that I'm planning to replace) were originally created in tablespace BIGTABLESPACE

-- 50G bigfile datafile size  
create bigfile tablespace "BIGTABLESPACE"  
datafile '$oracle\_home/oradata/$oracle\_sid/bts01.dbf' size 50G  
extent management local  
segment space management auto;  

The permanent table indexes were originally created in tablespace BIGTABLESPACE

-- 20G bigfile datafile size  
create bigfile tablespace "BIGINDXSPACE"  
datafile '$oracle\_home/oradata/$oracle\_sid/btsindx01.dbf' size 20G  
extent management local  
segment space management auto;  

• Is replacing these permanent tables with global temporary tables feasable?
• The TEMP tablespace will run into a problem extending the TEMP tablespace. Is there a way to create global temporary tables and their indexes in tablespaces BIGTABLESPACE and BIGINDXSPACE?
• If not, how can I make the TEMP tablespace behave like a bigfile tablespace and achieve index/table separation?
• Can I create two TEMP bigfile tablespaces and create indexes into one and tables into another?

I want to use global temporary tables, but the volume of data I am handling in the procedure would seem to be above and beyond the indended design of global temporary tables. Any suggestions?

Answer 1

I don't think that there's anything in your description that makes GTT's unattractive. You obviously need very large temporary tablespaces but you're not consuming more space overall unless you've been making heavy use of table compression (unavailable in GTT's at least up to 10gR2). Look into the use of tablespace groups: http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/tspaces.htm#ADMIN01103

Using a tablespace group, rather than a single temporary tablespace, can alleviate problems caused where one tablespace is inadequate to hold the results of a sort, particularly on a table that has many partitions. A tablespace group enables parallel execution servers in a single parallel operation to use multiple temporary tablespaces.

Also, don't neglect the use of subquery factoring clauses. They can often replace the use of temporary tables. However they might still require just as much temporary storage space because a large result set from a SQFC can spill to disk to avoid the consumption of too much memory, so you still have to go ahead with the increase in TEMP space. They're very handy for not having to deploy a new database object every time you need a new temporary table.

Answer 2

I looked at large sized Global Temporary Tables for a migration exercise. It worked but for debugging and rejection hadling I eventually went with plain tables.

If the GTTs don't work out, consider either Row-Level Security / VPD (or even views). You can have a column derived from sys_context('USERENV','SESSIONID') and use that to ensure that the user can only see their own data.

Still the thought of multiple sessions dealing with multi-gigabyte datasets concurrently is a bit scary.

PS. I believe that for GTTs used through a procedure use the temp tablespace of the session user rather than the temp tablespace of the procedure owner. If you can get the sessions as separate oracle users then you have a chance at spreading your file IO over different tablespaces.

Answer 3

There is no benefit to separating data and indexes into separate tablespaces other than potentially making DBAs more comfortable that similar objects are grouped together. There is a long-standing myth that separating indexes and data was beneficial for performance reasons-- that is not correct.

Temporary objects should (and must) be stored in a temporary tablespace. You could increase the size of your TEMP tablespace or create a separate temporary tablespace just for the user(s) that will own these objects if you wanted to segregate these large temporary tables into a separate tablespace. You can't (and wouldn't want to) store them in your permanent tablespaces.

Architecturally, though, I would be very curious about why temporary tables were necessary in your system. If you have sessions that are writing 10's of GB into temporary tables, then presumably reading those 10's of GB out again in order to write the data somewhere else, I would tend to suspect that there were more efficient solutions. It is very rare in Oracle to even need temporary tables-- it is far more common in other databases where readers can block writers to need to copy data out of tables before working on it. Oracle has no such limitations.