In a PostgreSQL db I'm working on, half of the tables have one particular column, always named the same, that is of type varchar(5). The size became a bit too restricting and I want to change it to varchar(10).
The number of tables in my particular case is actually very manageable to do it by hand. But I was wondering how one could script this with a query for larger dbs. It generally should be possible in just a few steps.
Identify all the tables in the schema, then (?) filter by condition if column present.
Create ALTER TABLE statements for each table found
I have some idea about how to write a query that identifies all tables in the schema. But I wouldn't know how to filter them. And if I didn't filter them, I assume the generated alter table statements would break.
Would be great if someone could share their knowledge on this.
Thanks to Abelisto for providing some guidance. Eventually, this is how I did it.
First, I created a query that in turn creates the ALTER TABLE statements. MyDB and MyColumn need to reflect actual values.
SELECT
'ALTER TABLE '||columns.table_name||' ALTER COLUMN '||MyColumn||' TYPE varchar(20);'
FROM
information_schema.columns
WHERE
columns.table_catalog = 'MyDB' AND
columns.table_schema = 'public' AND
columns.column_name = 'MyColumn';
Then it was just a matter of executing the output as a new query. All done.
Related
I have create a redshift cluster and created a db inside.
My schema is new_schema
I have created 2 tables inside two tables inside table1, table2
My Question.
I want to list the datatypes of table1
I need to change the datatype of description which is inside the table1 which is of VARCHAR to TEXT
I have tried to list the datatypes of table1 with below query but nothing listing
SELECT * FROM PG_TABLE_DEF WHERE schemaname = 'new_schema';
A few possibilities as to why you are not seeing the expected results. Most likely is that new_schema isn't in your search_path. Pg_table_info only return info for tables in your search_path - see: https://docs.aws.amazon.com/redshift/latest/dg/r_PG_TABLE_DEF.html
Another possibility is that the tables have no data rows (no blocks assigned) and this can lead to incomplete info from some system tables.
Another possibility is that the tables were not committed by the creating session and being checked by a different session. Since you say that you are creating a new db this comes to mind.
Are the tables visible in svv_table_info?
Also the premise of changing varchar to text is a bit off. From https://docs.aws.amazon.com/redshift/latest/dg/r_Character_types.html#r_Character_types-text-and-bpchar-types
You can create an Amazon Redshift table with a TEXT column, but it is
converted to a VARCHAR(256) column that accepts variable-length values
with a maximum of 256 characters.
So it seems like the objective you are trying to achieve is a bit off.
I have a parent table (parent_table) and few children tables inherit from it (child_one_table and child_two_table).
I want to query data using columns belong to the parent table only (the data itself is inserted to the children tables), when I run an explain on my query I see that there are sequence scans running on all the tables (parent_table, child_one_table and child_two_table).
Is there a more efficient way to do this? When I try to query using ONLY on parent_table I get back 0 result since the data was inserted to children table.
Thanks in advance!
Thanks for clarifying your question!
When I query the data for SELECT * FROM parent_table WHERE name = 'joe'; I see that it actually go over all 3 tables and query for that.
This seems to be standard behaviour from PostgreSQL. According to the 5.10.1. Caveats section of https://www.postgresql.org/docs/current/ddl-inherit.html
Note that not all SQL commands are able to work on inheritance hierarchies. Commands that are used for data querying, data modification, or schema modification (e.g., SELECT, UPDATE, DELETE, most variants of ALTER TABLE, but not INSERT or ALTER TABLE ... RENAME) typically default to including child tables and support the ONLY notation to exclude them.
So really what you want to do is explore the usage of ONLY to specify the scope of your query.
I hope this can help!
I have many tables in different databases and want to bring them to a database.
It seems like I have to create foreign table in the database (where I want to merge them all) with schemas of all the tables.
I am sure, there is a way to automate this (by the way, I am going to use psql command) but I do not know where to start.
what I have found so far is I can use
select * from information_schema.columns
where table_schema = 'public' and table_name = 'mytable'
I added more detail explanation.
I wanted to copy tables from another database
the tables have same column names and data type
using postgres_fdw, I needed to set up a field name and data type for each tables (the table names are also same)
then, I want to union the tables have same name all to have one single table.
for that, I am going to add prefix on table
for instance, mytable in db1, mytable in db2, mytable in db3 as in
db1_mytable, db2_mytable, db3_mytable in my local database.
Thanks to Albe's comment, I managed it and now I need to figure out doing 4th step using psql command.
I was simply wondering if there was a performance or other technical reason for you to be unable to perform a ALTER TABLE ALTER COLUMN statement with multiple columns within the same line e.g.
ALTER TABLE tblGeneric ALTER COLUMN Generic1 VARCHAR(255), Generic2 VARCHAR(255);
This is exclusively a restriction of t-sql as you can in fact comma separate columns with the MODIFY statement of mysql.
I just thought it odd, especially considering the MODIFY of mysql, that you can do a same line multi ALTER TABLE ADD statement but not a same line multi ALTER. I was just wondered if there is any particular documented reason for this or at least if it's in a issues list.
SQL Server T-SQL doesn't allow multiple columns to be changed in one ALTER TABLE command (unlike some other languages where it's possible).
Please follow this MSDN link for ALTER command syntax and explanation.
However, you can do multiple ADD or multiple DROP COLUMN, but just one ALTER COLUMN.
My database has severals table with some column type 'money'. I would like to alter all these columns (in different tables) in a single statement rather than change type column by column, to avoid omissions.
You'll have to repeat the altering query for every column.
You might want to create a program code to do that for you. You know, with loops.
In order for the database to alter all the tables atomically you should enclose all the altering queries in a transaction (PostgreSQL supports transactional DDL).