Tag: index

“= 123” vs. “= ‘depesz'”. What is faster?

There is this idea that normal form in databases require you to use integer, auto incrementing, primary keys.

The idea was discussed by many people, I will just point you to series of three blog posts on the subject by Josh Berkus ( part 1, 2 and 3, and reprise).

One of the points that proponents of surrogate keys (i.e. those based on integer and sequences) raise is that comparing integers is faster than comparing texts. So,

SELECT * FROM users WHERE id = 123

is faster than

SELECT * FROM users WHERE username = 'depesz'

Is it?

Continue reading “= 123" vs. “= ‘depesz'". What is faster?

Waiting for 9.2 – Index only scans

On 8th of October, Tom Lane committed patch:

Support index-only scans using the visibility map to avoid heap fetches.
 
When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.
 
There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.
 
Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.

Continue reading Waiting for 9.2 – Index only scans

What index to create?

Some time ago I wrote a blogpost about why index might not be used.

While this post seemed to be well received (top link from depesz.com on reddit), it doesn't answer another question – what index to create for given situation.

I'll try to cover this question now.

IMPORTANT UPDATE: As of PostgreSQL 10 hash indexes are WAL logged. As such, main point against them is gone.

Continue reading What index to create?

Waiting for 9.1 – Per-column collation support

On 8th of February, Peter Eisentraut committed patch:

Per-column collation support
 
This adds collation support for columns and domains, a COLLATE clause
to override it per expression, and B-tree index support.
 
Peter Eisentraut
reviewed by Pavel Stehule, Itagaki Takahiro, Robert Haas, Noah Misch
 
Branch
------
master

Continue reading Waiting for 9.1 – Per-column collation support

Waiting for 9.1 – Add UNIQUE/PRIMARY KEY with index

On 25th of January, Tom Lane committed patch:

Implement ALTER TABLE ADD UNIQUE/PRIMARY KEY USING INDEX.
 
This feature allows a UNIQUE OR pkey CONSTRAINT TO be created USING an
already-existing UNIQUE INDEX.  While the CONSTRAINT isn't very
functionally different from the bare index, it's nice TO be able TO do that
FOR documentation purposes.  The main advantage OVER just issuing a plain
ALTER TABLE ADD UNIQUE/PRIMARY KEY IS that the INDEX can be created WITH
CREATE INDEX CONCURRENTLY, so that there IS NOT a long INTERVAL WHERE the
TABLE IS locked against updates.
 
ON the way, refactor SOME OF the code IN DefineIndex() AND index_create()
so that we don't have to pass through those functions in order to create
the index constraint's catalog entries.  Also, IN parse_utilcmd.c, pass
around the ParseState pointer IN struct CreateStmtContext TO save ON
notation, AND ADD error location pointers TO SOME error reports that didn't
have one before.
 
Gurjeet Singh, reviewed by Steve Singer and Tom Lane

Continue reading Waiting for 9.1 – Add UNIQUE/PRIMARY KEY with index

Waiting for 9.1 – KNNGIST

On 4th of December, Tom Lane committed really cool patch:

KNNGIST, otherwise known as order-by-operator support for GIST.

This commit represents a rather heavily editorialized version of
Teodor's builtin_knngist_itself-0.8.2 and builtin_knngist_proc-0.8.1
patches.  I redid the opclass API to add a separate Distance method
instead of turning the Consistent method into an illogical mess,
fixed some bit-rot in the rbtree interfaces, and generally worked over
the code style and comments.
 
There's still no non-code documentation to speak of, but I'll work on
that separately.  Some contrib-module changes are also yet to come
(right now, point <-> point is the only KNN-ified operator).
 
Teodor Sigaev and Tom Lane

Continue reading Waiting for 9.1 – KNNGIST