We are thinking about to create new schema with its own 3 tables which will be created on the fly for an individual customer.
To run a particular query for those tables in a procedure, should we have something like this.
declare #sName nvarchar(200);
select #sName =Schema_Name from schema where Schema_Id = passed_id_from_code
ALTER USER UserName WITH DEFAULT_SCHEMA = #sName
-- Run the statements here --
...
-- After finishing executing statements
ALTER USER UserName WITH DEFAULT_SCHEMA = db;
In this scenario, can concurrent customers from various schema can update their own schema table or it will conflict.
Your suggestions are welcome.
Anil
Most SQL databases have each table create as an unique entity in that database. That means that each table can be modified and altered individually with no relation to the other tables. CUSTOMERA.TABLE_ONE is a different object in the database that CUSTOMERB.TABLE_ONE. They do share the same name, but they are not the same object with potentially different layouts (as they have different schemas).
So unless there is some restriction on the RDBMS you can do this. Now having different schemas for each user may not be good. If you are develop the same app to handle several customers, you have to make sure it will work with all schemas and all custoemrs. In potentially different versions of the schema.
If you are going for a multi-tenant architecture, it may be wiser to use some kind of extension to to table. So you have a single DB.TABLE_ONE, with a CUSTOMER_DATA column where you put data in a know and flexible format (say JSON or XML). Some RDBMS have that that as a native features (I believe DB2 is one of them).
Hope this helps.
Related
I have some questions about the schema in a table.
Sometime when you create a table the default schema is dbo.TableName. Is the dbo the default schema name? I believe you can change or specify the schema when creating a table right, because there are tables that have different schema like: Sales.Tablename or Users.Roles, etc. I believe the purpose of a schema is to make a difference between tables or something like that? Something like a namespace within a C# class. Is it possible to have two tables with same name but a different schema, like: Sales.Users, Marketing.Users ?
dbo is the default schema. You can change the default schema for each sql-login.
If you accidentally create a table in the wrong schema, you can move it:
-- Moving Peter table from Sales schema to Orders schema
ALTER SCHEMA Orders TRANSFER Sales.peter
You can specify which schema to create the table in by specifying it before the table name:
CREATE TABLE Sales.Users(id int);
One of the purposes of schemas is to create logical groups of tables, just like namespaces in C#. They are also useful for controlling permissions and more.
Yes, table names only need to be unique within each schema..
Sometime when you create a table the default schema is dbo.TableName. Is the dbo
the default schema name?
Why do you ask? It is quite obvious that dbo is the default schame name if you get it as default, or? On top it is the only usable schema a new database has.
I believe you can change or specify the schema when creating a table right,
What sense would multiple schemata have if you could not use them? And as the create table syntax clearly states you can specify a schema.
I believe the purpose of a schema is to make a difference between tables or
something like that? Something like a namespace within a C# class.
That pretty much sums it up.
Is it possible to have two tables with same name but a different schema,
What about you spend 10 seconds to try it out? Are you challenged by he concept of trying something totally simplistic out? And the answer is yes. object names have to be unique - within their schema.
Following is my schema detail:
DB_A : schema_1, schema_2, schema_3
DB_B : schema_3
some procedures in schema_3 access resources(table, view, sp) from schema_1 and schema_2.
All procedures in schema_3 are same on both the dbs. How do I access schema_1 from schema_3 for both the dbs.
Now I can hard code DB_A in my procedures but when I move code to client machine, it will create a problem since DB_A may not be same(one of the reason being client is miser and having QA, Dev and Prod on same machine).
Second option is getting DB_A name as a parameter, but it will make all the schema_3 SPs dynamic (as I did not get any method to access something like #DBName.schema_name.ResourceName).
Third option is creating linked servers, which again do not solve my problem because of same reason as first.
Any idea how to proceed, where I do not want my procedures to be dynamic just because 80% of them are straight.
Edit Start:
So I can restate it as I have multiple databases with a database having resources (table/view/schema) which needs to be shared and then having other databases (one or more) which have stored procedures which computes on data from shared database and self database.
Shared database name is not going to be constant on all the environments and I want to change them(environment specific). I have come out with a solution where I will be creating synonym for all the shared resources and all procedures will be using them, that way they are all referring to shared resources from first database.
For each installation I need to modify synonyms definition to reflect correct share database name. Is there any SYNONYM For Database Name, that way I will have way less synonyms to handle.
Well the best choice I found is as follows.
Create Synonym (independent database DB_B) for individual objects (in shared database DB_A) with same name in same schema. That way your existing procedures need not change, and will work as required. Synonym gives a good reference on this. I will soon be creating an app to ease creating synonyms for these kind of situations.
CREATE SYNONYM DB_B.schema_1.proc_1 FOR DB_A.schema_1.proc_1
You can run your procedure in DB_A and create a view from DB_A to DB_B:
create view dbo.vw_B_Schema_3
as
select *
from DB_B.dbo.Schema_3
You'd have to create three versions of the view (dev, QA, prod.) But the view will be the only difference: procedure definitions can remain identical.
If DB_A and DB_B are on same server, only sure you that the login have permission in two database.
Now, use [database].[schema].[object], when you use object of others database
eg: I have two database, ("helpdesk", "intranet")
from heldesk to intranet
create view dbo.users
as
select login, name, lastname
from intranet.dbo.user // [database].[schema].[object] user is a table in dbo schema from intranet database.
where status = 1
;
I have a Rails 3.2 multi-tenant subdomain based app which I'm trying to migrate over to PostgreSQL's schemas (each account getting its own schema -- right now all of the accounts use the same tables).
So, I'm thinking I need to:
Create a new DB
Create a schema for each Account (its id) and the tables under them
Grab all the data that belongs to each account and insert it into the new DB under the schema of said account
Does that sound correct? If so, what's a good way of doing that? Should I write a ruby script that uses ActiveRecord, plucks the data, then inserts it (pretty inefficient, but should get the job done) into the new DB? Or does Postgres provide good tools for doing such a thing?
EDIT:
As Craig recommended, I created schemas in the existing DB. I then looped through all of the Accounts in a Rake task, copying the data over with something like:
Account.all.each do |account|
PgTools.set_search_path account.id, false
sql = %{INSERT INTO tags SELECT DISTINCT "tags".* FROM "tags" INNER JOIN "taggings" ON "tags"."id" = "taggings"."tag_id" WHERE "taggings"."tagger_id" = #{admin.id} AND "taggings"."tagger_type" = 'User'}
ActiveRecord::Base.connection.execute sql
#more such commands
end
I'd do the conversion with SQL personally.
Create the new schemas in the same database as the current one for easy migration, because you can't easily query across databases with PostgreSQL.
Migrate the data using appropriate INSERT INTO ... SELECT queries. To do it without having to disable any foreign keys, you should build a dependency graph of your data. Copy the data into tables that depend on nothing first, then tables that depend on them, and so on.
You'll need to repeat this for each customer schema, so consider creating a PL/PgSQL function that uses EXECUTE ... dynamic SQL to:
Create the schema for a customer
Create the tables within the schema
Copy data in the correct order by looping over a hard-coded array of table names, doing:
EXECUTE `'INSERT INTO '||quote_ident(newschema)||'.'||quote_ident(tablename)||' SELECT * FROM oldschema.'||quote_ident(tablename)||' WHERE customer_id = '||quote_literal(customer_id)'||;'
where newschema, tablename and customer_id are PL/PgSQL variables.
You can then invoke that function from SQL. While you could do just select convert_customer(c.id) FROM customer GROUP BY c.id, I'd probably do it from an external control script just so each customer's work got done and committed individually, avoiding the need to start again from scratch if the second-to-last customer conversion fails.
For bonus crazy points it's even possible to define triggers on the main customer schema's tables that replicate changes to already-migrated customers over to the copy of their data in the new schema, so they can keep using the system during the migration. I'd avoid that unless the migration was just too big to do without downtime, as it'd be a nightmare to test and you'd still need the triggers to throw an error on access by customer id x while the migration of x's data was actually in-progress, so it wouldn't be fully transparent.
If you're using different login users for different customers (strongly recommended) your function can also:
REVOKE rights on the schema from public
GRANT limited rights on the schema to the user(s) or role(s) who'll be using it
REVOKE rights on public from each table created
GRANT the desired limited rights on each table to the user(s) and role(s)
GRANT on any sequences used by those tables. This is required even if the sequence is created by a SERIAL pseudo-column.
That way all your permissions are consistent and you don't need to go and change them later. Remember that your webapp should never log in as a superuser.
I did a succesful migration from MySql to Sql Server using the migration tool.
Unfortunately for some reason it labels the tables database.DBO.tablename instead of just database.tablename
I have never used Sql Server so perhaps this is just the way they name their tables.
When I do:
SELECT TOP 1000 [rid]
,[filename]
,[qcname]
,[compound]
,[response]
,[isid]
,[isidresp]
,[finalconc]
,[rowid]
FROM [test].[calibration]
it does not work
But, when I do:
SELECT TOP 1000 [rid]
,[filename]
,[qcname]
,[compound]
,[response]
,[isid]
,[isidresp]
,[finalconc]
,[rowid]
FROM [test].[dbo].[calibration]
it works.
Does anyone know why it prefixes with DBO?
dbo is the standard database owner for anything you create (tables, stored procedures, etc,..), hence the migration tool automatically prefixing everything with it.
When you access something in Sql Server, such as a table called calibration, the following are functionally equivalent:
calibration
dbo.calibration
database_name.dbo.calibration
server_name.database_name.dbo.calibration
MySql doesn't, as far as I remember (we migrated a solution from MySql to SqlServer about 12 months ago using custom scripts executed by nant) support database owner's when referencing objects, hence you're probably not familiar with four part (server_name.database_name.owner_name.object_name) references.
Basically, if you want to specify the database you're accessing, you also need to specify the "owner" of the object. i.e, the following are functionally identical:
USE [master]
GO
SELECT * FROM [mydatabase].[dbo].[calibration]
USE [mydatabase]
GO
SELECT * FROM [calibration]
SqlServer uses an owner name when it references tables. In this case, dbo is the owner.
MySQL doesn't use owner for table names, which is why you didn't see those names before.
SQL Server has something called schemas, in this case the default schema is dbo but it could be anything you wanted. Schemas are used to logically group objects. So you can create a Employee schema and have all the Employee tables, views, procs and functions in there, this then also enables you to give certain users only access to certain schemas
Tell me your migration tool you have used, and let me know the version of from and to databases.
Regards
Eugene
You do have an issue here with the default schema, if it's set to 'dbo' for the user you logged in as you don't need to specify it. See http://msdn.microsoft.com/en-us/library/ms176060.aspx
I'm going to guess that the answer is "no" based on the below error message (and this Google result), but is there anyway to perform a cross-database query using PostgreSQL?
databaseA=# select * from databaseB.public.someTableName;
ERROR: cross-database references are not implemented:
"databaseB.public.someTableName"
I'm working with some data that is partitioned across two databases although data is really shared between the two (userid columns in one database come from the users table in the other database). I have no idea why these are two separate databases instead of schema, but c'est la vie...
Note: As the original asker implied, if you are setting up two databases on the same machine you probably want to make two schemas instead - in that case you don't need anything special to query across them.
postgres_fdw
Use postgres_fdw (foreign data wrapper) to connect to tables in any Postgres database - local or remote.
Note that there are foreign data wrappers for other popular data sources. At this time, only postgres_fdw and file_fdw are part of the official Postgres distribution.
For Postgres versions before 9.3
Versions this old are no longer supported, but if you need to do this in a pre-2013 Postgres installation, there is a function called dblink.
I've never used it, but it is maintained and distributed with the rest of PostgreSQL. If you're using the version of PostgreSQL that came with your Linux distro, you might need to install a package called postgresql-contrib.
dblink() -- executes a query in a remote database
dblink executes a query (usually a SELECT, but it can be any SQL
statement that returns rows) in a remote database.
When two text arguments are given, the first one is first looked up as
a persistent connection's name; if found, the command is executed on
that connection. If not found, the first argument is treated as a
connection info string as for dblink_connect, and the indicated
connection is made just for the duration of this command.
one of the good example:
SELECT *
FROM table1 tb1
LEFT JOIN (
SELECT *
FROM dblink('dbname=db2','SELECT id, code FROM table2')
AS tb2(id int, code text);
) AS tb2 ON tb2.column = tb1.column;
Note: I am giving this information for future reference. Reference
I have run into this before an came to the same conclusion about cross database queries as you. What I ended up doing was using schemas to divide the table space that way I could keep the tables grouped but still query them all.
Just to add a bit more information.
There is no way to query a database other than the current one. Because PostgreSQL loads database-specific system catalogs, it is uncertain how a cross-database query should even behave.
contrib/dblink allows cross-database queries using function calls. Of course, a client can also make simultaneous connections to different databases and merge the results on the client side.
PostgreSQL FAQ
Yes, you can by using DBlink (postgresql only) and DBI-Link (allows foreign cross database queriers) and TDS_LInk which allows queries to be run against MS SQL server.
I have used DB-Link and TDS-link before with great success.
I have checked and tried to create a foreign key relationships between 2 tables in 2 different databases using both dblink and postgres_fdw but with no result.
Having read the other peoples feedback on this, for example here and here and in some other sources it looks like there is no way to do that currently:
The dblink and postgres_fdw indeed enable one to connect to and query tables in other databases, which is not possible with the standard Postgres, but they do not allow to establish foreign key relationships between tables in different databases.
If performance is important and most queries are read-only, I would suggest to replicate data over to another database. While this seems like unneeded duplication of data, it might help if indexes are required.
This can be done with simple on insert triggers which in turn call dblink to update another copy. There are also full-blown replication options (like Slony) but that's off-topic.
see https://www.cybertec-postgresql.com/en/joining-data-from-multiple-postgres-databases/ [published 2017]
These days you also have the option to use https://prestodb.io/
You can run SQL on that PrestoDB node and it will distribute the SQL query as required. It can connect to the same node twice for different databases, or it might be connecting to different nodes on different hosts.
It does not support:
DELETE
ALTER TABLE
CREATE TABLE (CREATE TABLE AS is supported)
GRANT
REVOKE
SHOW GRANTS
SHOW ROLES
SHOW ROLE GRANTS
So you should only use it for SELECT and JOIN needs. Connect directly to each database for the above needs. (It looks like you can also INSERT or UPDATE which is nice)
Client applications connect to PrestoDB primarily using JDBC, but other types of connection are possible including a Tableu compatible web API
This is an open source tool governed by the Linux Foundation and Presto Foundation.
The founding members of the Presto Foundation are: Facebook, Uber,
Twitter, and Alibaba.
The current members are: Facebook, Uber, Twitter, Alibaba, Alluxio,
Ahana, Upsolver, and Intel.
In case someone needs a more involved example on how to do cross-database queries, here's an example that cleans up the databasechangeloglock table on every database that has it:
CREATE EXTENSION IF NOT EXISTS dblink;
DO
$$
DECLARE database_name TEXT;
DECLARE conn_template TEXT;
DECLARE conn_string TEXT;
DECLARE table_exists Boolean;
BEGIN
conn_template = 'user=myuser password=mypass dbname=';
FOR database_name IN
SELECT datname FROM pg_database
WHERE datistemplate = false
LOOP
conn_string = conn_template || database_name;
table_exists = (select table_exists_ from dblink(conn_string, '(select Count(*) > 0 from information_schema.tables where table_name = ''databasechangeloglock'')') as (table_exists_ Boolean));
IF table_exists THEN
perform dblink_exec(conn_string, 'delete from databasechangeloglock');
END IF;
END LOOP;
END
$$