Tips N’ Tricks – Generating readable reports with plain SQL

Let's say you imported some data, but it contains duplicates. You will have to handle them in some way, but to make sensible choice on how to handle it, you need more information.

So, let's start. We have table:

# \d users
                                    Table "public.users"
   Column   |           Type           |                     Modifiers
 id         | integer                  | not null default nextval('users_id_seq'::regclass)
 username   | text                     |
 registered | timestamp with time zone |
    "users_pkey" PRIMARY KEY, btree (id)

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Set operations in shell

I had this interesting case at work. We have imports of objects. Each object in import file has its “ID" (which can be any string). Same “ID" is in database.

So the idea is pretty simple – we can/should check how many of IDs from import were in database. Unfortunately – we'd rather not really do the comparison in DB, as it is pretty loaded.

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Getting list of most common domains

Today, on #postgresql on IRC, guy (can't contact him now to get his permission to name him), said:

I have a table called problematic_hostnames. It contains a list of banned hostnames in column “hostname" (varchar). I would like to display the top 10 troll ISPs based on this. Does PG have a way of spotting a “pattern"? Some ISPs are while others are, so you can't just regexp the last X.Y (since that would cause “" to be one of the top troll ISPs).

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Waiting for 8.4 – sql-wrappable RETURNING

In PostgreSQL 8.2, we got “RETURNING" clause in INSERT/UPDATE/DELETE queries.

Unfortunately it could not be used as source of rows for anything in sql.

INSERT INTO table_backup DELETE FROM TABLE WHERE ... returning *;

Well, it's still not possible, but it is a one step closer, thanks to patch written and committed by Tom Lane on 31st of October:

RETURNING clause, NOT just a SELECT AS formerly.
A side effect OF this patch IS that WHEN a set-returning SQL FUNCTION IS used
IN a FROM clause, performance IS improved because the output IS collected INTO
a tuplestore WITHIN the FUNCTION, rather than USING the less efficient
value-per-CALL mechanism.

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Text comparisons that does automatic trim()

SoftNum asked on irc:

< SoftNum> does postgresql have a config option to automatically trim (both ' ' from blah) on string compares?

So, can you?

Of course there is no such option, but maybe there is a way to tell PostgreSQL to do this trim for given field? Sure there is 🙂

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