Monday, June 24, 2013

Let me SLEEP!

DBMS_LOCK is a slightly obscure built-in package. It provides components which so we build our own locking schemes. Its obscurity stems from the default access on the package, which is restricted to its owner SYS and the other power user accounts. Because implementing your own locking strategy is a good way to wreck a system, unless you really know what you're doing. Besides, Oracle's existing functionality is such that there is almost no need to need to build something extra (especially since 11g finally made the SELECT ... FOR UPDATE SKIP LOCKED syntax legal). So it's just fine that DBMS_LOCK is private to SYS. Except ...

... except that one of the sub-programs in the package is SLEEP(). And SLEEP() is highly useful. Most PL/SQL applications of any sophistication need the ability to pause processing for a short while, either a fixed time or perhaps polling for a specific event. So it is normal for PL/SQL applications to need access to DBMS_SLEEP.LOCK().

Commonly this access is granted at the package level, that is grant execute on dbms_lock to joe_dev. Truth to be told, there's not much harm in that. The privilege is granted to a named account, and if somebody uses the access to implement a roll-your-own locking strategy which brings Production to its knees, well, the DBAs know who to look for.

But we can employ a schema instead. The chief virtue of a schema is managing rights on objects. So let's create a schema for mediating access to powerful SYS privileges:

create user sys_utils identified by &pw
temporary tablespace temp
/
grant create procedure, create view, create type to sys_utils
/

Note that SYS_UTILS does not get the create session privilege. Hence nobody can connect to the account, a sensible precaution for a user with potentially damaging privileges. Why bar connection in all databases and not just Production? The lessons of history tell us that developers will assume they can do in Production anything they can do in Development, and write their code accordingly.

Anyway, as well as granting privileges, the DBA user will need to build SYS_UTIL's objects on its behalf:
grant execute on dbms_lock to sys_utils
/
create or replace procedure sys_utils.sleep
    ( i_seconds in number)
as
begin
    dbms_lock.sleep(i_seconds);
end sleep;
/
create public synonym sleep for sys_utils.sleep
/
grant execute on sys_utils.sleep to joe_dev
/

I think it's a good idea to be proactive about creating an account like this; granting it some obviously useful privileges before developers ask for them, simply because some developers won't ask. The forums occasionally throw up extremely expensive PL/SQL loops whose sole purpose is to burn CPU cycles or wacky DBMS_JOB routines which run every second. These WTFs have their genesis in ignorance of, or lack of access to, DBMS_LOCK.SLEEP().

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Oracle 10g - a time traveller's tale

Time travel sucks, especially going back in time. Nobody takes a bath, there are no anaesthetics and you can't get a decent wi-fi signal anywhere. As for killing your own grandfather, forget about it.

The same is true for going back in database versions. In 2009 I had gone straight from an Oracle 9i project to an Oracle 11g one. So when I eventually found myself on a 10g project it was rather disorientating. I would keep reaching for tools which weren't in the toolbox: LISTAGG(), preprocessor scripts for external tables, generalized invocation for objects.

I had missed out on 10g while it was shiny and new, and now it just seemed limited. Take Partitioning. Oracle 10g supported exactly the same composite partitioning methods as 9i: just Range-hash and Range-List, whereas 11g is full of wonders like Interval-Range, Hash-Hash and the one I needed, List-List.

Faking a List-List composite partitioning scheme in 10g

Consider this example of a table with a (slightly forced) need for composite List-List partitioning. It is part of a engineering stock control system, in which PRODUCTS are grouped in LINES (Ships, Cars, Planes) and COMPONENTS are grouped into CATEGORIES (Frame, interior fittings, software, etc). We need an intersection table which links components to products.

There are hundreds of thousands of components and tens of thousands of products. But we are almost always only interested in components for a single category within a single product line (or product) so composite partitiong on (product_line, component_category) is a good scheme. In 11g the List-List method works just fine:
SQL> create table product_components
  2      (product_line varchar2(10) not null
  3          , product_id number not null
  4          , component_category varchar2(10) not null
  5          , component_id number not null
  6          , constraint pc_pk primary key (product_id, component_id )
  7          , constraint pc_prd_fk foreign key (product_id )
  8             references products (product_id)
  9          , constraint pc_com_fk foreign key (component_id )
 10             references components (component_id)
 11      )
 12  partition by range(product_line) subpartition by list(component_category)
 13       subpartition template
 14           (subpartition sbody values ('BODY')
 15            , subpartition sint values ('INT')
 16            , subpartition selectr values ('ELECTR')
 17            , subpartition ssoft values ('SOFT')
 18           )
 19      (partition pship values ('SHIP')
 20       , partition pcar values  ('CAR')
 21       , partition pplane values ('PLANE')
 22       )
 23  /

Table created.

SQL> 

But in 10g the same statement hurls ORA-00922: missing or invalid option . The workaround is a bit of a nasty hack: replace the first List with a Range, producing a legitimate Range-List composite:
SQL> create table product_components
  2      (product_line varchar2(10) not null
  3          , product_id number not null
  4          , component_category varchar2(10) not null
  5          , component_id number not null
  6          , constraint pc_pk primary key (product_id, component_id )
  7          , constraint pc_prd_fk foreign key (product_id )
  8             references products (product_id)
  9          , constraint pc_com_fk foreign key (component_id )
 10             references components (component_id)
 11      )
 12  partition by range(product_line) subpartition by list(component_category)
 13       subpartition template
 14           (subpartition sbody values ('BODY')
 15            , subpartition sint values ('INT')
 16            , subpartition selectr values ('ELECTR')
 17            , subpartition ssoft values ('SOFT')
 18           )
 19      (partition pcar values less than ('CAS')
 20       , partition pplane values less than ('PLANF')
 21       , partition pship values less than ('SHIQ')
 22       )
 23  /

Table created.

SQL> 

Note the wacky spellings which ensure that 'CAR' ends up in the right partition. Also we have to re-order the partition clause so that the partition bounds don't raise an ORA-14037 exception. We are also left with the possibility that a rogue typo might slip records into the wrong partition, so we really ought to have a foreign key constraint on the product_line column:
alter table product_components add constraint pc_prdl_fk foreign key (product_line) 
           references product_lines (line_code)
/

I described this as a nasty hack. It is not really that nasty, in fact it actually works very well in daily processing. But managing the table is less intuitive. Say we want to manufacture another line, rockets. We cannot just add a new partition:
SQL> alter table product_components 
    add partition prock values less than ('ROCKEU')
/
  2    3      add partition prock values less than ('ROCKEU')
                  *
ERROR at line 2:
ORA-14074: partition bound must collate higher than that of the last partition


SQL> 

Instead we have to split the PSHIP partition in two:
SQL> alter table product_components split partition pship
  2     at ('ROCKEU')
  3     into (partition  prock, partition pship)
  4  /

Table altered.

SQL> 

The other snag is, that once we do get back to the future it's a bit of a chore to convert the table to a proper List-List scheme. Probably too much of a chore to be worth the effort. Even with a time machine there are only so many hours in the day.

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Friday, June 21, 2013

Where's SCOTT?

The database on the Developer Days Database App VBox appliance doesn't have the SCOTT schema. This is fair enough, as the sample schemas aren't include by default any more (for security reasons, obviously). I know the official sample schemas used in the documentation - HR, OE, and so on - are more realistic and useful for developing prototypes. But nothing beats the simplicity of SCOTT.EMP for explaining something in an online forum.

So, where is the script for building the SCOTT schema?

Back in the day it was part of the SQL*Plus build: $ORACLE_HOME/sqlplus/demo/demobld.sql (or something, I'm doing this from memory). But in 11gR2 there are no demo scripts in the sqlplus sub-directory. This was also probably the case in 10g but I never had occasion to look for it on that platform. Anyway, in 11gR2 its location is $ORACLE_HOME/admin/rdbms/utlsampl.sql.

Presumably Oracle have obscured it because they want everybody to stop using SCOTT and standardise on the modern samples. But this schema is part of the Oracle heritage. It should have Grade II listed status.

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