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From "Tomohito Nakayama (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-252) polishing up idea of default value including auto increment of a column in table
Date Sun, 01 May 2005 02:20:05 GMT
polishing up idea of default value including auto increment of a column in table

         Key: DERBY-252
         URL: http://issues.apache.org/jira/browse/DERBY-252
     Project: Derby
        Type: Task
  Components: Documentation  
    Reporter: Tomohito Nakayama
    Priority: Minor

 This is not bug. However, there exists some complexity in the part of default value and autoincrement
value in current implementation of derby.
 Take aside whether any modification to be done or not for this issue, polishing up the idea
of default including autoincrement would be help thinking about default and autoincrement.

Next is my idea for default...

 "default value of the column" is value which registered to the column when a record was inserted
without any value.

 "default value of the column" can be one of the next.
  def_1: default does not exist
  def_2: constant value
  def_3: generated by a rule
  def_3-1: autoincrement value
  def_3-2: value generated by more complex rule than autoincrement (This does not exists in

 There exists question wheter def_3 is truly default or not.

 In the current implementation of derby, object interfaced as "org.apache.derby.catalog.DefaultInfo"
was externalized to "COLUMNDEFAULT" of "SYS_COLUMNS" in the case of def_2, and the object
have information of what constant value is used as default value.
 On the other hand, information about autoincrement value was stored to "IDENTITYCOLUMNVALUE",

 There exists question in the case of def_3, why information about autoincrement value does
not exist in DefaultInfo.
 One answer for this question is that storing information in "IDENTITYCOLUMNVALUE" to DefaultInfo
would cause performance trouble, because this will cause re-storing all information of DefaultInfo
always when record was inserted and next autoincrement value was prepared. 
 But this answer does not explan about information of "IDENTITYCOLUMNSTART", "IDENTITYCOLUMNINC".

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