"ineffective" is an understatement. Inserting a value that is bigger
than 1/2 of a page will cause the insert to fail - with some sort of key
too big error. Current btree mechanism does not support keys bigger
than 1/2 of a page.
Many systems get around this by only indexing the first N bytes/chars (I
think 512 bytes is the mySQL value of N), of the datatype. Derby does
not have this support, but could be implemented. Building and
maintaining such an index would be easy. Some work would have to be
done in the language layer to teach it that the value in the index key
could only be used for indexing into the actual value in the base row,
it could never be used as a covering key. No changes would be necessary
at the store btree level, as it does not really know what it is storing.
Satheesh Bandaram (JIRA) wrote:
[ http://issues.apache.org/jira/browse/DERBY-392?page=all ]
Satheesh Bandaram updated DERBY-392:
------------------------------------
Description:
I guess I did not articulate my reasons for suggesting removal of index support for 'long varchar for bit data' completely.
1) Long varchar types are not comparable... If they are not comparable, it should not be possible to use them in GROUP BY, ORDER BY or allow regular B-Tree indexes.
2) Also, long varchar types tend to be long in size and hence the regular
B-Tree mechanism is not a suitable way to index them. Dan also mentioned they
become ineffective for keys longer than half a page size.
It should not be possible to create an index on 'long varchar for bit data'
datatypes. Derby currently doesn't allow creating indexes on 'long varchar'
datatypes and the same should apply for it's bit data equivalent too.
ij> create table longchar ( i int, c long varchar);
0 rows inserted/updated/deleted
ij> create index longIdx on longchar(c);
ERROR X0X67: Columns of type 'LONG VARCHAR' may not be used in CREATE INDEX, ORD
ER BY, GROUP BY, UNION, INTERSECT, EXCEPT or DISTINCT statements because compari
sons are not supported for that type.
ij> create table longcharBitData ( i int, c long varchar for bit data);
0 rows inserted/updated/deleted
ij> create index longIdx on longcharBitData(c);
0 rows inserted/updated/deleted
Derby also seems to allow GROUP BY and/or ORDER BY on LONG VARCHAR FOR BIT DATA
types. I believe this is incorrect too.
select c from longcharBitData group by c;
C
--------------------------------------------------------------------------------
------------------------------------------------
0 rows selected
ij> select c from longcharBitData group by c order by c;
C
--------------------------------------------------------------------------------
------------------------------------------------
0 rows selected
was:
It should not be possible to create an index on 'long varchar for bit data'
datatypes. Derby currently doesn't allow creating indexes on 'long varchar'
datatypes and the same should apply for it's bit data equivalent too.
ij> create table longchar ( i int, c long varchar);
0 rows inserted/updated/deleted
ij> create index longIdx on longchar(c);
ERROR X0X67: Columns of type 'LONG VARCHAR' may not be used in CREATE INDEX, ORD
ER BY, GROUP BY, UNION, INTERSECT, EXCEPT or DISTINCT statements because compari
sons are not supported for that type.
ij> create table longcharBitData ( i int, c long varchar for bit data);
0 rows inserted/updated/deleted
ij> create index longIdx on longcharBitData(c);
0 rows inserted/updated/deleted
Disable creating indexes on long varchar for bit data. Long varchar column
doesn't allow creating indexes already.
------------------------------------------------------------------------------------------------------------------
Key: DERBY-392
URL: http://issues.apache.org/jira/browse/DERBY-392
Project: Derby
Type: Bug
Components: SQL
Versions: 10.0.2.2, 10.1.1.0
Environment: generic
Reporter: Satheesh Bandaram
Priority: Minor
I guess I did not articulate my reasons for suggesting removal of index support
for 'long varchar for bit data' completely.
1) Long varchar types are not comparable... If they are not comparable, it
should not be possible to use them in GROUP BY, ORDER BY or allow regular
B-Tree indexes.
2) Also, long varchar types tend to be long in size and hence the regular
B-Tree mechanism is not a suitable way to index them. Dan also mentioned they
become ineffective for keys longer than half a page size.
It should not be possible to create an index on 'long varchar for bit data'
datatypes. Derby currently doesn't allow creating indexes on 'long varchar'
datatypes and the same should apply for it's bit data equivalent too.
ij> create table longchar ( i int, c long varchar);
0 rows inserted/updated/deleted
ij> create index longIdx on longchar(c);
ERROR X0X67: Columns of type 'LONG VARCHAR' may not be used in CREATE INDEX, ORD
ER BY, GROUP BY, UNION, INTERSECT, EXCEPT or DISTINCT statements because compari
sons are not supported for that type.
ij> create table longcharBitData ( i int, c long varchar for bit data);
0 rows inserted/updated/deleted
ij> create index longIdx on longcharBitData(c);
0 rows inserted/updated/deleted
Derby also seems to allow GROUP BY and/or ORDER BY on LONG VARCHAR FOR BIT DATA
types. I believe this is incorrect too.
select c from longcharBitData group by c;
C
--------------------------------------------------------------------------------
------------------------------------------------
0 rows selected
ij> select c from longcharBitData group by c order by c;
C
--------------------------------------------------------------------------------
------------------------------------------------
0 rows selected
