Forgive my ignorance, but I'm wondering if there is a way to specify metadata for a table in PostgreSQL that I don't want it to be as a field in that table. For instance, if I want to add a Description field for that table, creation Time, etc...
I know I can do this using extra tables, but I'd prefer having not to do this, to be honest. I've digged in the official PostgreSQL docs, but there's nothing there besides looking in information_schema.tables, where I guess I'm not allowed to modify anything.
Any clues? Otherwise, I guess I'll have to create a few more tables to handle this.
Thanks!
There's the comment field:
COMMENT ON TABLE my_table IS 'Yup, it's a table';
In current versions the comment field is limited to a single text string. There's been discussion of allowing composite types or records, but AFAIK no agreement on any workable design.
You can shove JSON into the comments if you want. It's a bit dirty, since it'll show up as the Description column in \d+ output in psql, etc, but it'll work.
craig=> COMMENT ON TABLE test IS 'Some table';
COMMENT
craig=> \d+
List of relations
Schema | Name | Type | Owner | Size | Description
--------+----------------------+----------+-------+------------+-------------
public | test | table | craig | 8192 bytes | Some table
You can get the comment from SQL with:
SELECT pg_catalog.obj_description('test'::regclass, 'pg_class');
Comments can also be added on other objects, like columns, data types, functions, etc.
If that doesn't fit your needs you're pretty much stuck with a side table for metadata.
People regularly request table metadata like creation time, etc, but nobody tends to step up with a workable plan for an implementation and the time and enthusiasm to carry it through to the finish. In any case the most common request is "last modified time", which is pretty horrible from a performance point of view and difficult to get right in the face of multi-version concurrency control, transaction isolation rules, etc.
Related
I am unsure how to design security policies for a following system including counters in postgres/supabase. My database includes two tables:
Users:
uuid|name|follower_counter
------------------------------
xyz |tobi| 1
Following-Relationship
follower| following
---------------------------
uuid_1 | uuid_2
Once a user follows a different user, I would like to use a postgres function/transaction to
Insert a new following-follower relationship
Update the followed users' counter
BEGIN
create follower_relationship(follower_id, following_id);
update increment_counter_of_followed_person(following_id);
END;
The constraint should be that the users table (e.g. the name column) can only be altered by the user owning the row. However, the follower_counter should open to changes from users who start following that user.
What is the best security policy design here? Should I add column security or should exclude the counters to a different table?
Do I have to pass parameters to the "block transaction" to ensure that the update and insert functions are called with the needed rights? With which rights should I call the block function?
It might be better to take a different approach to solve this problem. Instead of having a column dedicated to counting the followers, I would recommend actually counting the number of followers when you query the users. Since you already have Following-Relationship table, we just need to count the rows within the table where following or follower is the querying user.
When you have a counter, it might be hard to keep the counter accurate. You have to make sure the number gets decremented when someone unfollows. What if someone blocks a user? What if a user was deleted? There could be a lot of situations that could throw off the counter.
If you count the number of followings/followers on the fly, you don't need to worry about those situations at all.
Now obvious concern with this approach that you might have is performance, but you should not worry too much about it. Postgres is a powerful database that has been battle tested for decades, and with a proper index in place, it can easily perform these query on the fly.
The easiest way of doing this in Supabase would be to create a view like this the following. Once you create a view, you can query it from your Supabase client just like a typical table!
create or replace view profiles as
select
id,
name,
(select count(*) from following_relationship where followed_user_id = id) as follower_count,
(select count(*) from following_relationship where following_user_id = id) as following_count
from users;
We have a people table, each person has a gender defined by a gender_id to a genders table,
| people |
|-----------|
| id |
| name |
| gender_id |
| genders |
|---------|
| id |
| name |
Now, we want to allow people to create forms by themselves using a nice form builder. One of the elements we want to add is a select list with user defined options,
| lists |
|-------|
| id |
| name |
| list_options |
|--------------|
| id |
| list_id |
| label |
| value |
However, they can't use the genders as a dropdown list because it's in a different table. They could create a new list with the same options as genders but this isn't very nice and if a new gender is added they'd need to add it in multiple places.
So we want to move the gender options into a list that the user can edit at will and will be reflected when a new person is created too.
What's the best way to move the genders into a list and list_options while still having a gender_id (or similar) column in the people table? Thoughts I've had so far include;
Create a 'magic' list with a known id and always assume that this contains the gender options.
Not a great fan of this because it sounds like using 'magic' numbers. The code will need some kind of 'map' between system level select boxes and what they mean
Instead of having a 'magic' list, move it out into an option that the user can choose so they have a choice which list contains the genders.
This isn't really much different, but the ID wouldn't be hardcoded. It would require more work looking through DB tables though
Have some kind of column(s) on the lists table that would mark it as pulling its options from another table.
Would likely require a lot more (and more complex) code to make this work.
Some kind of polymorphic table that I'm not sure how would work but I've just thought about and wanted to write down before I forget.
No idea how this would work because I've only just had the idea
The easiest solution would change your list_options table to a view. If you have multiple tables you need have a list drop down for to pull from this table, just UNION result sets together.
SELECT
(your list id here) -- make this a part primary key
id, -- and this a part primary key
Name,
FROM dbo.Genders
UNION
SELECT
(your list id here) -- make this a part primary key
id, -- and this a part primary key
Name,
FROM dbo.SomeOtherTable
This way it's automatically updated anytime the data changes. Now you are going to want to test this, as if this gets big it might get slow, you can get around this by only pulling all this information once in your application (or say cache it for 30 minutes and then refresh just in case).
Your second option is to create a table list_options and then create a procedure (etc.) which goes through all the other lookup tables and pulls the information to compile it. This will be faster for application performance, but it will require you to keep it all in sync. The easiest way to handle this one is to create a series of triggers which will rebuild portions (or the entire) list_options table when something in the look up tables is changed. In this one, I would suggest moving away from creating a automatically generated primary key and move to a composite key, like I mentioned with the views. Since this is going to be rebuilt, the id will change, so it's best to not having anything think that value is at all stable. With the composite (list_id,lookup_Id) it should always be the same no matter how many times that row is inserted into the table.
I have a table with several fields, this table almost never change but for one field, "version" which change very often.
Would it be relevant to put that single field into a separate table in order to reduce how often locks are put on the main table?
For instance I have a table tType and a table tEntry.
Whenever I add/deleted/update any row of tEntry, I need to update the "version" field of tType. There might be thousand of rows inside tEntry for a single tType referenced row. Meaning the version number could change very often, though any other data of tType (such as name, id, etc.) doesn't change.
Your Referral to tType and tEntry sounds like you are implementing a key-value store in a rdbms. There are several discussions you can google about this topic. In the www there seems to be consesus, that cons overweight pros on that. An option would be to look at key value stores, no sql, multi column DBs, etc (wikipedia)...
The next "anti-pattern" I recognized is that you try to mix transactional data with 'master data' in the table tType. Try to avoid this, even if your selects get more uncomfortable and need to be tuned better. Keep off the version info from the tType, if this changes extremely often. Look here to get the concept: MySQL JOIN the most recent row only?
Hopefully an easy question for someone with more experience than me. I have a stored procedure that Inserts records into a table. Like all databases that I have worked with, when you insert a record it inserts it into the bottom of the table. I would like to insert it to the top of the table and then move all the existing records down by one (I assume this would happen automatically with the insert).
I want to to do this because I'm using the 'Top #' keyword. I am pretty sure that I could just leave it the way it is, and instead of using the 'Top" keyword, I could use the 'Bottom" keyword. But I want to make it easier for people reading it that aren't familiar with it, so they can instantly see the newest entries. I'm going to keep researching this on my own, but If someone knew off the top of their head and could save me the time that would be appreciated.
is there any incremental id on that table.If yes then create clustered index on that id with descending order
I'm interested in using the following audit mechanism in an existing PostgreSQL database.
http://wiki.postgresql.org/wiki/Audit_trigger
but, would like (if possible) to make one modification. I would also like to log the primary_key's value where it could be queried later. So, I would like to add a field named something like "record_id" to the "logged_actions" table. The problem is that every table in the existing database has a different primary key fieldname. The good news is that the database has a very consistent naming convention. It's always, _id. So, if a table was named "employee", the primary key is "employee_id".
Is there anyway to do this? basically, I need something like OLD.FieldByName(x) or OLD[x] to get value out of the id field to put into the record_id field in the new audit record.
I do understand that I could just create a separate, custom trigger for each table that I want to keep track of, but it would be nice to have it be generic.
edit: I also understand that the key value does get logged in either the old/new data fields. But, what I would like would be to make querying for the history easier and more efficient. In other words,
select * from audit.logged_actions where table_name = 'xxxx' and record_id = 12345;
another edit: I'm using PostgreSQL 9.1
Thanks!
You didn't mention your version of PostgreSQL, which is very important when writing answers to questions like this.
If you're running PostgreSQL 9.0 or newer (or able to upgrade) you can use this approach as documented by Pavel:
http://okbob.blogspot.com/2009/10/dynamic-access-to-record-fields-in.html
In general, what you want is to reference a dynamically named field in a record-typed PL/PgSQL variable like 'NEW' or 'OLD'. This has historically been annoyingly hard, and is still awkward but is at least possible in 9.0.
Your other alternative - which may be simpler - is to write your audit triggers in plperlu, where dynamic field references are trivial.