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Database Migration

This document describes the steps to be taken in order to migrate an Oracle database to UTF8. It is based on Oracle 9i using the WE8ISO8859P15 (Latin 15) character set, but in princip should apply to other configurations as well.

Inhalt

Data Cleaning

Before we go for the migration, we have to clean our database. This is being done mainly for the following reasons:

This is just to mention a few items and shows, that besides being helpful for the migration process, data cleaning will also have positive aspect on the every day maintenance and work. Thus, if there are data in our database which cannot be used or will not be used anymore: This is a good time to get rid of it. Even if you don't plan to migrate at all …

Identify objects to be deleted

Your database probably contains tables which are no longer used and thus could be deleted after an optional export. These tables can be grouped into the categories

Moreover there may be complete schematas which are not being used anymore, and can be grouped by the same categories.

Once identified confidentally, these objects should be backed up (e.g. using Oracles EXP utility) – just in case they may be needed again later on - and then finally dropped.

Data no more needed

In a next step we are looking for obsolete/outaged data. These may e.g. include

As with the objects above, these data should be backed up and deleted from the database.

Data Integrity

Verifying the database schema logic, you may find out that many "foreign key constraints" are missing and where obviously more or less handled from the application only. Checking these may bring up a list of orphans, where the application failed to resolve the referenced data, leaving alone the bones we just can delete (due to the missing reference, they cannot be of any use).

To find all those "bones", you need to know your database model well. Another indicator may be constraints which are either not enabled at all (check if these make sense in your logic – if so, they should not be disabled; if not, they should be dropped), or enabled but not validated. The latter ones are pretty good indicators: You should try to validate them, and check all violations – which will most likely be some of those "bones".

To prevent these orphans in the future, the missing "foreign key constraints" should be established. Before doing so on the database, the applications need to be checked for possible side-effects (e.g. if the app tries to do the logic, it might run into some constraint violations when the foreign keys are established). The required application changes then need to be published at the same time as the constraints are established, to avoid problems.

Problematic objects

In parallel to the missing foreign key constraints discussed above, there are a couple of more objects we should check for to avoid possible problems. These include:

Below you will find an SQL script to list up all these possible troublemakers. For some of these objects this is an important job, since e.g. triggers may have been disabled for a good reason (and should not be "accidentally" enabled after the migration), others may just indicate problems in the database or the application. So do a careful investigation on them.

--
-- Discover possible problematic objects
--
-- Author: AI Rehberg
-- $LastChangedDate: 2007-08-31 15:04:37 +0200 (Fr, 31 Aug 2007) $

-- disabled triggers
select *
  from dba_triggers
 where status != 'ENABLED'
   and owner not in 'SYS'
 order by owner

-- Invalid objects (packages,views,...)
select owner,object_name "Object",object_type "OType",status,temporary "Temp"
  from dba_objects
 where status != 'VALID'
   and owner not in ('SYS')
 order by owner

-- Possible broken packages/procedures/views/...
-- (may give detail information on above selected broken objects
select distinct owner,name,type,line,position,text
  from dba_errors
 where text not like '%Statement ignored%'
 order by owner,name

-- Disabled constraints
select owner,constraint_name,constraint_type "CType",table_name,status,validated,last_change
  from dba_constraints
 where 1=1
   and status !='ENABLED'
   and owner not in ('SYS','SYSTEM','WMSYS')
 order by owner

-- Not validated constraints
select owner,constraint_name,constraint_type "CType",table_name,status,validated,last_change
  from dba_constraints
 where 1=1
   and validated!='VALIDATED'
   and owner not in ('SYS','SYSTEM','WMSYS')
 order by owner

-- Indices with outdated or no statistics
select owner,index_name,index_type "IType",table_name,uniqueness,tablespace_name,logging "Log",status,last_analyzed,temporary "Temp"
  from dba_indexes
 where 1=1
   and owner not in ('SYS','SYSTEM','WMSYS','CSMIG','CTXSYS','MDSYS','ORDSYS','OUTLN','TOAD','XDB')
   and (last_analyzed is null or last_analyzed < add_months(sysdate,-2))
 order by owner

-- Invalid indices
select owner,index_name,index_type "IType",table_name,uniqueness,tablespace_name,logging "Log",status,last_analyzed,temporary "Temp"
  from dba_indexes
 where 1=1
   and owner not in ('SYS','SYSTEM','WMSYS','CSMIG','CTXSYS','MDSYS','ORDSYS','OUTLN','TOAD','XDB')
   and status != 'VALID'
 order by owner

Character Set Cleaning

Mainly two different problems are addressed by this topic, resulting in a double-check on the database plus maybe changes in the environment:

  1. Wrong characters in the database
  2. Characters to be converted

The first issue, where applicable, was most likely introduced due to wrong environment setups. The NLS_LANG variable should reflect the client's system character set. Instead, on some machines it may reflect the internal character set of the database (here: 'WE8ISO8859P15'). Although this may sometimes match, it will cause a problem on the Windows clients, which use the 'WE8MSWIN1252' (Windows-1252) character set. As a result, the Win1252-characters from their input will be introduced to the database without the necessary character set translation - so we have Win1252 characters in our Latin-15 database. Since in the other direction (the client is reading from the database) no conversion took place either, these characters will still be displayed correctly there, while garbage will be shown to clients with correct setup. Converting the database without correcting these characters first would result in a data loss, since the characters in question would not be converted correctly (but instead all show up as question marks – real ones or upside-downs).

The second issue is just the result of our goal: Since we want to migrate to a different character set (i.e. 'AL32UTF8'), which luckily is a super-set of the character set used right now, for some characters the "character code" (i.e. the position/number in the character table) will change, requiring a migration of existing content.

How to solve the wrong characters

As stated above, without fixing this problem some data would not be converted correctly: The Win1252 characters in the database would just be replaced by a "dummy placeholder". The solution consists of three steps:

  1. Identify the data in question
  2. Replace the wrong characters
  3. Correct the environment settings (i.e. set NLS_LANG=WE8MSWIN1252 on the windows clients)

The first step is done using Oracles Character Set Scanner (CSScan). To identify our problem candidates, we pretend that we want to convert to WE8ISO8859P15, using this for both the "from" and "to" character set – this way CSScan will report all data which are not in Latin1, in our case the Win1252 data. There's no way to automatically replace the wrong characters since Latin1 is not a super-set of the Win1252 character set.

This means rows found "not convertible" need to be manually corrected. Before this is done, we cannot continue with the next step.

Characters to be converted

When all "wrong characters" are fixed (see previous paragraph), i.e. when the database contains only characters according to its specified character set – and not before that, we can check for its convertibility. This process is similar to the previous one, except that we set the target character set to AL32UTF8. As a result, our data will be divided into up to 4 categories:

  1. Changeless: Nothing will change here
  2. Convertible: The necessary conversions will be done automatically
  3. Truncation: The necessary conversions could be done automatically, but the resulting string would be too long to fit into the column
  4. Lossy: Due to "wrong characters" (see above), data loss is imminent

Hopefully, most data belongs to category 1 and 2 and only few (or better none) to category 3. Candidates for category 4 should have been eliminated with the previous step – if there are any of these reported here, return to there.

The data for the 3rd category need special attention. If any data falls into this category, we need to check our table definitions and adjust the affected column size accordingly – otherwise the data listed in this category will be truncated on conversion, causing a data loss. If that's not possible (i.e. the column size is already the maximum supported by Oracle), we need to truncate these data manually or automatically. This applies only to those columns which – as a result of the conversion – would have to hold more than 4.000 bytes – so don't worry about the e.g. VARCHAR2(50) fields reported: The CSScan tool just sees the current state, which says "50 (bytes)". We will later change the semantics for all these fields from BYTE to CHAR, which the CSScan tool cannot know at this stage. So you only need to be worried about fields longer than 2.000: If those are completely filled with characters needing 2 bytes in UTF8, then you will have a problem. We come back to this issue later.

Preparing the database structures

In most cases, the migration process will require setting up a new database already using the AL32UTF8 character set – the exceptions where you simply could ALTER DATABASE CHARACTER SET AL32UTF8 are very rare. So what you will have to do is:

  1. Take the scripts used to create your current database, make a copy of them, and edit the copy concerning the character set to use. Also take care for the database name - you cannot use the same name if that instance will be running on the same machine like the original database.
  2. Create the new database. Make sure you set NLS_LENGTH_SEMANTICS=CHAR in the new databases environment before.
  3. Do a full export from the originating database with the ROWS=N parameter (this will simply export all objects like tables, triggers and stored procedures - but no data)
  4. Take that export file, and import the schemata you want to migrate into the new UTF8 database (do not import the SYS, SYSTEM, etc. schematas which ship with Oracle, they are already there!)

Since all these 4 steps belong to the DBAs daily duties, they won't be explained in depth at this place – I rather focus on the real issues ;)

NLS Length Semantics

By default, the NLS_LENGTH_SEMANTICS is set to BYTE. This means, if you declare a column VARCHAR2(50), Oracle implicitly makes that VARCHAR2(50 BYTES). What does that mean? Nothing to your old Latin-15 database, since here each character uses exactly one byte. But with your UTF8 database, it can mean a lot – if you are not only store US7ASCII characters (why would you want an UTF8 database for that?). As for those, UTF8 also uses 1 byte per character. But e.g. for the accentuated French/Spanish/… characters, or the German Umlauts, UTF8 will use 2 bytes. So with the default settings, you are unable to say how much characters fit into that column.

To avoid this problem, we need to set the NLS_LENGTH_SEMANTICS=CHAR. Then, Oracle interprets our above column implicitely to VARCHAR2(50 CHARS), and everything is fine. Hopefully. There still remains one limit for the VARCHAR2 columns, which explicitely states "4.000 bytes". So you of course can define a column VARCHAR2(4000 CHARS) – but with UTF8 this does not necessarily mean that always 4.000 chars fit in there – see above...

Semantic conversions

So you set up the NLS_LENGTH_SEMANTICS correctly, created the UTF8 database and imported the structures. You thing why not directly import the data along, right? Maybe you already guessed it: Your imported structures kept the state they have been created with, i.e. they are still using byte semantics. This was explicitely written into the export dump file, so whom to blame …

This leaves us with the next task: We have to convert all this to use character semantics. Luckily, we can use some script to do the work for us, and don't have to check everything manually. But Oracle doesn't deliver such a script. Time for me to mention my IZ_MIGRATION package the first time, which contains the necessary procedures. The package is downloadable for free at IzzySoft.DE.

So you can simply use the iz_migration.semantics2char procedure to do the job for you (for details, please see the documentation of that package). You may also consider to convert all your national character fields, such as NCHAR or NVARCHAR to their "normal pendants" – the package also contains procedures for that.

Field truncation

While talking about the character set cleaning and the Oracle provided CSScan tool, we mentioned rows that would be truncated and the need to manually check those. If you want to postpone this job (or automate it and don't care about the cut-off), you can use the iz_migration.truncate_long_fields procedure for that. It will only cut-off what really has to be cut-off, while storing the original content in a special tables CLOB field together with the object name and row id, so it is easily identifyable for later review. If you don't truncate the fields beforehand, you will run into problems on data import, since these rows would be skipped with an error message. This way you at least can keep what is possible, and fix the details later.

Data transfer: Different strategies compared

There are different ways to migrate the data themselves. First, a table will give a brief overview – later the strategies will be discussed into more detail:

Strategy Pros Cons
ALTER DATABASE Fast and safe Only applyable in very rare cases
EXP/IMP Safe and supported by Oracle Slow, requires long downtime
DBLink Copy Minimal downtime Not supported by Oracle, not so error-proof

The ALTER DATABASE method

If this method can be applied, it is for sure first choice: From all the methods described here, it is not only the fastest (requires almost no downtime), but it also is supported by Oracle. Sounds very good so far - so there must be a rub in it. Right, there is: It won't work if you start off from a Latin-15 database. So it's out of discussion here.

The Export/Import method

So what is Oracle recommending us in our case? Right: Make a complete, consistent export and import it into your new UTF8 database (basically). Details you will find on all Oracle pages, so no need to discuss that here. And I guess you already see the hitch: To make a complete and consistent export, you have to close your database for a while. Same for the import of that dump, for even a longer time. And you probably don't want anybody change the data in the original database while you're importing into the new one, since you want be up-to-date when opening the new instance - so add both times together. You will easily come up with one day off for everybody who depends on that database, since you need 10 hours or more for the process …

Not always it is affordable to close the database for an entire workday. Not even if you could provide a read-only access for that time in some way. So we have been looking for some alternative. Surprisingly, copying an entire table via a database link turned out to be much faster than an export/import, so I investigated closer on that – the result was the IZ_MIGRATION package.

The idea behind it is simple: Since we already imported the entire structure into the new database, we can simply walk through the data dictionary and do a "for each owner – for each table – insert into owner.table select * from owner.table@dblink". There are some more things to take care for in this context (all explained in the packages documentation), including:

For the constraint and trigger issue, the package contains appropriate scripts to disable and enable them. As for the speed, constraints we will only ENABLE NOVALIDATE – assuming the data have been validated before, we just need to make sure that no new "violating" entries can be inserted. An additional procedure to validate them can be run later at any time.

Dropping the indexes also can be done with a procedure of the package. Before doing so, you will need to extract a script to re-create them later – you will find that script at the end of this page. The sequences, again, will be catered for by a procedure from the IZ_MIGRATION package.

Warning concerning the DBLink Copy Method

As already stated above, the DBLink copy method is not supported nor even recommended by Oracle. Even worse, it is not that much tested yet. I ran it already successfully on a few migrations, and thereby fixed the one or the other issue – but that does not mean it can be recommended to migrate any production database. The instances I migrated up to now using the package, all had very much in common – and since I created the package I exactly know what it is doing, and thus can solve potential trouble somehow. So if you really want to use this method, you should either hire me along for that task, or at least run everything thoroughly with a test system (which is as close as possible to your production database) to see that the result is really what it should be! Best, of course, is to combine both ;)

Script to generate the CREATE INDEX statements

-- Retrieving all index information (create-script) for non-pk and non-unique indexes
SET SERVEROUTPUT ON FEEDBACK OFF
SPOOL create_indexes.sql
DECLARE
  town VARCHAR2(40) := '0';
  CURSOR cur IS
    SELECT i.owner,i.index_name
      FROM dba_indexes i,dba_constraints c
     WHERE i.owner = c.owner(+)
       AND i.index_name = c.index_name(+)
       AND c.index_name IS NULL
       AND i.index_type NOT LIKE 'IOT%'
       AND i.index_type != 'LOB'
       AND i.owner NOT IN ('SYS','SYSTEM','CSMIG','CTXSYS','DBSNMP','DMSYS','EXFSYS',
                           'HR','IX','MDSYS','OE','OLAPSYS','ORDSYS','OUTLN',
                           'SH','SYSMAN','WKSYS','WK_TEST','WMSYS','XDB')
     ORDER BY i.owner,i.index_name;
  PROCEDURE print(line IN VARCHAR2) IS
    -- wrapper to dbms_output.put_line - work around the 255 characters limit
    pos NUMBER;
    FUNCTION strpos (str IN VARCHAR2,needle IN VARCHAR2,startpos NUMBER) RETURN NUMBER IS
      -- split string helper to procedure print
      pos NUMBER; strsub VARCHAR2(255);
      BEGIN
        strsub := SUBSTR(str,1,255);
        pos    := INSTR(strsub,needle,startpos);
        return pos;
      END;
    BEGIN
      dbms_output.put_line(line);
    EXCEPTION
      WHEN OTHERS THEN
        IF SQLERRM LIKE '%ORU-10028%' THEN
          pos := strpos(line,' ',-1);
	  print(SUBSTR(line,1,pos));
	  pos := pos +1;
	  print(SUBSTR(line,pos));
	ELSE
      dbms_output.put_line('-- *!* Problem in print() *!*');
	END IF;
    END;
BEGIN
  dbms_output.enable(1000000);
  FOR rec IN cur LOOP
    IF town != rec.owner THEN
      town := rec.owner;
      print('-- ===============================[ Schema '||town||' ]===');
    END IF;
    print(dbms_metadata.get_ddl('INDEX',rec.index_name,rec.owner)||';');
  END LOOP;
END;
/
SPOOL OFF

Verdict

There are different strategies to migrate a database to UTF8 – from "very safe" to "safe but faster", we discussed them a little bit in detail. Many steps in the preparation phase are common to all strategies.

Whatever strategy you chose, the IZ_MIGRATION package can assist you and make some otherwise a bit difficult tasks a lot easier. You may want to check the IzzySoft.DE website for more details on this and other helpful Oracle packages and software.

2018-12-16