polishing up idea of default value including auto increment of a column in table
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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 derby)
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", "IDENTITYCOLUMNSTART", "IDENTITYCOLUMNINC" of
"SYS_COLUMNS".
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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