Foreign key constraint backing index creation can be smarter when foreign key
is a subset of the table's primary key
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Key: DERBY-2204
URL: https://issues.apache.org/jira/browse/DERBY-2204
Project: Derby
Issue Type: Improvement
Components: Performance, SQL
Reporter: Daniel John Debrunner
When a foreign key's columns is a subset (or matches) the table's primary key
then there is no need to create an additional backing index. Such an index will
occupy disk space and decrease insert/update/delete performance.
Two typical situations where this is a problem are shown in the schema for the
oe system test.
1) A multiple column primary key has a sub-set of columns which are a primary
key in another table.
ALTER TABLE CUSTOMER ADD CONSTRAINT
CUSTOMER_PK PRIMARY KEY(C_W_ID, C_D_ID, C_ID);
ALTER TABLE CUSTOMER ADD CONSTRAINT
C_D_FK_DISTRICT FOREIGN KEY (C_W_ID,C_D_ID) REFERENCES DISTRICT;
Derby will create two backing indexes here, one for the primary key and one for
the foreign key. Derby could be improved
so that no index is created for the foreign key, since Derby can perform an
index scan on a sub-set of the first N columns in an index.
Not sure how easy fixing the runtime portion of constraint manipulation is, if
the constraint enforcement is via compiled SQL queries then it should be easier
than if they are direct scans against the access api.
2) A primary key in one table matches a primary key in another table, typically
when the "sub" table contains additional optional information related to the
main table.
ALTER TABLE NEWORDERS ADD CONSTRAINT
NEWORDERS_PK PRIMARY KEY(NO_W_ID, NO_D_ID, NO_O_ID);
ALTER TABLE NEWORDERS ADD CONSTRAINT
NO_O_FK FOREIGN KEY (NO_W_ID, NO_D_ID, NO_O_ID) REFERENCES ORDERS;
Derby will create two identical backing indexes here, one for the primary key
and one for the foreign key.
Fixing this case might be easier than 1) since Derby already supports having
mutliple logical indexes map to a single physical index.
In both cases work is needed to handle dropping of the primary key while the
foreign key constraint is kept:
- in case 1) a new index will need to be built using the sub-set of the
columns, thus dropping a primary key constraint could take time.
- in case 2) possibly the backing index can be re-used but it needs to be
converted from a unique index to a non-unique one.
Also the case where the primary key is created after the foreign key needs to
be covered, though that could be a follow on.
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