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CHAPTER 10 DATABASE TABLES
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The OID '<big hex number>' syntax is not documented in the Oracle documentation. All this is doing is ensuring that during an EXP and subsequent IMP, the underlying type PERSON_TYPE is, in fact, the same type. This will prevent an error that would occur if we performed the following steps: 1. 2. 3. 4. 5. Create the PEOPLE table. Export the table. Drop the table and the underlying PERSON_TYPE. Create a new PERSON_TYPE with different attributes. Import the old PEOPLE data.
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Obviously, this export cannot be imported into the new structure it will not fit. This check prevents that from occurring. If you remember, I mentioned that we can change the behavior of the object identifier assigned to an object instance. Instead of having the system generate a pseudo primary key for us, we can use the natural key of an object. At first, this might appear self-defeating the SYS_NC_OID$ column will still appear in the table definition in SYS.COL$ and, in fact, it will appear to consume massive amounts of storage as compared to the system-generated column. Once again, however, there is magic at work here. The SYS_NC_OID$ column for an object table that is based on a primary key and not system generated is a virtual column and consumes no real storage on disk. Here is an example that shows what happens in the data dictionary and demonstrates that there is no physical storage consumed for the SYS_NC_OID$ column. We ll start with an analysis of the systemgenerated OID table: ops$tkyte@ORA11GR2> create table people of person_type 2 / Table created. ops$tkyte@ORA11GR2> select name, type#, segcollength 2 from sys.col$ 3 where obj# = ( select object_id 4 from user_objects 5 where object_name = 'PEOPLE' ) 6 and name like 'SYS\_NC\_%' escape '\' 7 / NAME TYPE# SEGCOLLENGTH ------------------------- ---------- -----------SYS_NC_OID$ 23 16 SYS_NC_ROWINFO$ 121 1 ops$tkyte@ORA11GR2> insert into people(name) 2 select rownum from all_objects; 48217 rows created. ops$tkyte@ORA11GR2> exec dbms_stats.gather_table_stats( user, 'PEOPLE' ); PL/SQL procedure successfully completed. ops$tkyte@ORA11GR2> select table_name, avg_row_len from user_object_tables; TABLE_NAME AVG_ROW_LEN ------------------------------ ----------PEOPLE 23
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CHAPTER 10 DATABASE TABLES
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We see here that the average row length is 23 bytes: 16 bytes for the SYS_NC_OID$ column and 7 bytes for the NAME column. Now, let s do the same thing, but use a primary key on the NAME column as the object identifier: ops$tkyte@ORA11GR2> CREATE TABLE "PEOPLE" 2 OF "PERSON_TYPE" 3 ( constraint people_pk primary key(name) ) 4 object identifier is PRIMARY KEY 5 / Table created. ops$tkyte@ORA11GR2> select name, type#, segcollength 2 from sys.col$ 3 where obj# = ( select object_id 4 from user_objects 5 where object_name = 'PEOPLE' ) 6 and name like 'SYS\_NC\_%' escape '\' 7 / NAME TYPE# SEGCOLLENGTH ------------------------------ ---------- -----------SYS_NC_OID$ 23 81 SYS_NC_ROWINFO$ 121 1 According to this, instead of a small 16-byte column, we have a large 81-byte column! In reality, there is no data stored in there. It will be null. The system will generate a unique ID based on the object table, its underlying type, and the value in the row itself. We can see this in the following: ops$tkyte@ORA11GR2> insert into people (name) 2 values ( 'Hello World!' ); 1 row created. ops$tkyte@ORA11GR2> select sys_nc_oid$ from people p; SYS_NC_OID$ ------------------------------------------------------------------------------F610733A48F865F9E030007F0100149A00000017260100010001002900000000000C07001E01000 02A00078401FE000000140C48656C6C6F20576F726C642100000000000000000000000000000000 0000 ops$tkyte@ORA11GR2> select utl_raw.cast_to_raw( 'Hello World!' ) data 2 from dual; DATA ------------------------------------------------------------------------------48656C6C6F20576F726C6421 ops$tkyte@ORA11GR2> select utl_raw.cast_to_varchar2(sys_nc_oid$) data 2 from people; DATA ------------------------------------------------------------------------------<garbage bits and bytes..>Hello World!
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CHAPTER 10 DATABASE TABLES
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If we select out the SYS_NC_OID$ column and inspect the HEX dump of the string we inserted, we see that the row data itself is embedded in the object ID. Converting the object ID into a VARCHAR2, we can just confirm that visually. Does that mean our data is stored twice with a lot of overhead with it No, it is not it is just factored into that magic thing that is the SYS_NC_OID$ column upon retrieval. Oracle synthesizes the data upon selecting from the table. Now for an opinion. The object relational components (nested tables and object tables) are primarily what I call syntactic sugar. They are always translated into good old relational rows and columns. I prefer not to use them as physical storage mechanisms personally. There are too many bits of magic happening side effects that are not clear. You get hidden columns, extra indexes, surprise pseudo columns, and so on. This does not mean that the object-relational components are a waste of time. On the contrary, I use them in PL/SQL constantly. I use them with object views. I can achieve the benefits of a nested table construct (less data returned over the network for a master/detail relationship, conceptually easier to work with, and so on) without any of the physical storage concerns. That is because I can use object views to synthesize my objects from my relational data. This solves most of my concerns with object tables/nested tables in that the physical storage is dictated by me, the join conditions are set up by me, and the tables are available as relational tables (which is what many thirdparty tools and applications will demand) naturally. The people who require an object view of relational data can have it, and the people who need the relational view can have it. Since object tables are really relational tables in disguise, we are doing the same thing Oracle does for us behind the scenes, only we can do it more efficiently, since we don t have to do it generically as they do. For example, using the types defined earlier, I could just as easily use the following: ops$tkyte@ORA11GR2> 2 ( name 3 dob 4 home_city 5 home_street 6 home_state 7 home_zip 8 work_city 9 work_street 10 work_state 11 work_zip 12 ) 13 / Table created. create table people_tab varchar2(30) primary key, date, varchar2(30), varchar2(30), varchar2(2), number, varchar2(30), varchar2(30), varchar2(2), number
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ops$tkyte@ORA11GR2> create view people of person_type 2 with object identifier (name) 3 as 4 select name, dob, 5 address_type(home_city,home_street,home_state,home_zip) home_adress, 6 address_type(work_city,work_street,work_state,work_zip) work_adress 7 from people_tab 8 / View created. ops$tkyte@ORA11GR2> insert into people values ( 'Tom', '15-mar-1965', 2 address_type( 'Reston', '123 Main Street', 'Va', '45678' ), 3 address_type( 'Redwood', '1 Oracle Way', 'Ca', '23456' ) ); 1 row created.
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