Hi hackers,
I'm working on a patch that allows to combine covering and unique functionality for btree indexes.
Previous discussion was here: 1)
Proposal thread 2)
Message with proposal clarification In a nutshell, the feature allows to create index with "key" columns and "included" columns.
"key" columns can be used as scan keys. Unique constraint relates only to "key" columns.
"included" columns may be used as scan keys if they have suitable opclass.
Both "key" and "included" columns can be returned from index by IndexOnlyScan.
Btree is the default index and it's used everywhere. So it requires properly testing.
Volunteers are welcome)
Use case: - We have a table (c1, c2, c3, c4);
- We need to have an unique index on (c1, c2).
- We would like to have a covering index on all columns to avoid reading of heap pages.
Old way:
CREATE UNIQUE INDEX olduniqueidx ON oldt USING btree (c1, c2);
CREATE INDEX oldcoveringidx ON oldt USING btree (c1, c2, c3, c4);
What's wrong?
Two indexes contain repeated data. Overhead to data manipulation operations and database size.
New way:
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);
The patch is attached.
In 'test.sql' you can find a test with detailed comments on each step, and comparison of old and new indexes.
New feature has following syntax:
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);
Keyword INCLUDING defines the "included" columns of index. These columns aren't concern to unique constraint.
Also, them are not stored in index inner pages. It allows to decrease index size.
Results: 1) Additional covering index is not required anymore.
2) New index can use IndexOnlyScan on queries, where old index can't.
For example,
explain analyze select c1, c2 from newt where c1<10000 and c3<20;
*more examples in 'test.sql'
Future work: To do opclasses for "included" columns optional.
CREATE TABLE tbl (c1 int, c4 box);
CREATE UNIQUE INDEX idx ON tbl USING btree (c1) INCLUDING (c4);
If we don't need c4 as an index scankey, we don't need any btree opclass on it.
But we still want to have it in covering index for queries like
SELECT c4 FROM tbl WHERE c1=1000;
SELECT * FROM tbl WHERE c1=1000;
--
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
