IOTs and overflow tablespace

Question

Hi everyone,
I created an IOT without an overflow tablespace. What does Oracle do if it cannot fit a row into the index block? Do it give an error or will it use multiple index blocks to store the row of data?
Thanks,
Paul

Answer 1

<quote>I created an IOT without an overflow tablespace.
What does Oracle do if it cannot fit a row into the index block?</quote>
It will never happen.
An IOT defined with or without an overflow will get a default PCTTHRESHOLD of 50 ...
the allowed values are between 1 and 50.
PCTTHRESHOLD represents the percentage in an index block which could be occupied by one IOT row.
flip@FLOP>  show parameter db_block_size
NAME                                 TYPE        VALUE
db_block_size                        integer     8192
flip@FLOP>  create table tt
  2  ( x   number     not null
  3   ,c1  char(2000) not null
  4   ,c2  char(2000) not null
  5   ,constraint ttpk primary key (x)
  6  )
  7  organization index
  8  ;
create table tt
ERROR at line 1:
ORA-01429: Index-Organized Table: no data segment to store overflow row-pieces
So, Oracle sees we didn’t define an overflow and does some upfront checking on the declared size of the row
(BTW, it will be the same if the CHAR columns were defined as VARCHAR2).
It figures up it cannot store a row in 50% of my 8K block (there is some overhead in the block too).
flip@FLOP> create table tt
  2  ( x   number     not null
  3   ,c1  char(2000) not null
  4   ,constraint ttpk primary key (x)
  5  )
  6  organization index
  7  ;
Table created.
Not it can. And just to show, PCTHRESHOLD is defaulted to 50 …
flip@FLOP>  select dbms_metadata.get_ddl( 'TABLE', 'TT') from dual;
DBMS_METADATA.GET_DDL('TABLE','TT')
  CREATE TABLE "FLIP"."TT"
   (    "X" NUMBER NOT NULL ENABLE,
        "C1" CHAR(2000) NOT NULL ENABLE,
         CONSTRAINT "TTPK" PRIMARY KEY ("X") ENABLE
   ) ORGANIZATION INDEX NOCOMPRESS PCTFREE 10 INITRANS 2 MAXTRANS 255 LOGGING
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
  TABLESPACE "USERS"
PCTTHRESHOLD 50
So, if you have wide rows you better have an overflow … well, you have to.