Let's imagine following situation:
CREATE TABLE test (id int4 PRIMARY KEY, priority int4); INSERT INTO test (id) SELECT DISTINCT (random() * 100000000)::int4 FROM generate_series(1,1000);
Table test will now contain some (up to 1000) records, with random ids.
Now, we want to update first 3 records (ordered by id) to have following values in priority:
Continue reading Tips N’ Tricks – setting field based on order
Like practically any application, applications that I deal with evolve. They change their requirements when it comes to database storage – new tables, new columns, modified columns. Or perhaps – new base data – like new values in virtually static dictionaries.
For past years we've been working with set of pl/plgsql functions which kept track of “patches", and their dependencies.
The problem with this approach is that it doesn't really scale well, and it generates problems when we use replication (Slony).
The problems mean that we have to apply the patches “by hand" on master/slave server, because downtime in the day is not acceptable, and nobody is willing to do upgrades at 3 am just to be able to add new column.
So, we deal with it. But lately I grew annoyed by this, and started to think about a better way to organize patches.
I “dream" about a system when I would write a patch itself, and the system will make it possible to install and uninstall it with automatic changing database to prior state (this is not simple with things like “update", but it is definitely possible).
System which would “understand" replication, and apply changes to all replicated servers in a way that it will be as safe as possible with no or minimal downtime.
System which would let me track dependencies, and then install them if I'll tell it to install patch, that requires some other patches that were not applied to target database.
Basically – I want something like apt-get/dpkg/rpm for database.
So, writing this seems to be perfectly possible, but is it necessary? Perhaps somebody someplace already wrote such system? Do you know any? Or should I stop whining, sit down and write it myself?
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 🙂
Continue reading Text comparisons that does automatic trim()
Let's assume you have very simple table with users:
# \d users
Table "public.users"
Column | Type | Modifiers
-----------+---------+----------------------------------------------------
id | integer | not null default nextval('users_id_seq'::regclass)
username | text | not null
is_active | boolean | not null default true
Indexes:
"users_pkey" PRIMARY KEY, btree (id)
"users_username_key" UNIQUE, btree (username)And you'd like to count all users, and know how many of them are active …
Continue reading Tips N' Tricks – count of all and just some
On 4th of September Tom Lane committed another great patch. This one is very large, and even after applying – it's has some rough edges. There will be need for additional patches to make the functionality fully robust, but the fact that it got committed means that it will be available in final 8.4.
What does it do?
Continue reading Waiting for 8.4 – Common Table Expressions (WITH queries)
You might be familiar with hstore datatype in PostgreSQL – if you're not – check it out, as it is really cool.
Basically it's hash in database, which you can search in.
Continue reading Deserialization of hstore data structure in Perl
On 30th of September, Heikki Linnakangas committed his patch that changes FSM:
Rewrite the FSM. Instead of relying on a fixed-size shared memory segment, the free space information is stored in a dedicated FSM relation fork, with each relation (except for hash indexes; they don't use FSM). This eliminates the max_fsm_relations and max_fsm_pages GUC options; remove any trace of them from the backend, initdb, and documentation. Rewrite contrib/pg_freespacemap to match the new FSM implementation. Also introduce a new variant of the get_raw_page(regclass, int4, int4) function in contrib/pageinspect that let's you to return pages from any relation fork, and a new fsm_page_contents() function to inspect the new FSM pages.