I was wondering, if it is possible to have two temp tables with the same name in two separate queries without them conflicting when called upon later in the queries.
Query 1: Create Temp Table Tmp1 as ...
Query 2: Create Temp Table Tmp1 as ...
Query 1: Do something with Tmp1 ...
I am wondering if postgresql distinguishes between those two tables, maybe through addressing them as Query1.Tmp1 and Query2.Tmp1
Each connection to the database gets its own special temporary schema name, and temp tables are created in that schema. So there will not be any conflict between concurrent queries from separate connections, even if the tables have the same names. https://dba.stackexchange.com/a/5237 for more info
The PostgreSQL docs for creating tables states:
Temporary tables exist in a special schema, so a schema name cannot be given when creating a temporary table.
Related
I would like to obtain from database B, by querying its database tables, the original schemas and table names of its foreign tables (i.e. those in the original database A where they actually reside, as their name can be changed when created with the available fdw).
Any hint greatly appreciated!
Currently I am only able to find columns names and datatypes of the foreign tables (in database B) ...
I am reading Internals of postgreSQL chp 1 and I am unable to understand the difference between object identifier and relfilenode.
Tables and indexes as database objects are internally managed by individual OIDs, while those data files are managed by the variable, relfilenode. The relfilenode values of tables and indexes basically but not always match the respective OIDs
I get that both these are the attributes of the system catalog 'pg_class' and OID can be thought of as the primary key of the table, so what is the purpose of relfilenode and how is it different from OID?
relfilenode is the prefix for the name of the files that make up the table. Initially it is identical to the immutable object ID (oid), but SQL statements that rewrite the table will modify it (for example VACUUM (FULL), CLUSTER, TRUNCATE or the variants of ALTER TABLE that rewrite the table).
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.
In SQL stored procedures, we have an option of creating a temporary table "#temp" whose structure is as that of another table that it is referring to. Here we don't explicitly create and mention the structure of "#temp" table.
Do we have similar option is HQL Hive script to create a temp table during run time without actually creating the table structure. Thus I can dump data to temp table and use it. Below code shows an example of #temp table in SQL.
SELECT name, age, gender
INTO #MaleStudents
FROM student
WHERE gender = 'Male'
Hive has the concept of temporary tables, which are local to a user's session. These tables behave just like any other table, and can be created using CTAS commands too. Hive automatically deletes all temporary tables at the end of the Hive session in which they are created.
Read more about them here.
Hive Documentation
DWGEEK
You can create simple temporary table. On this table you can perform any operation.
Once you are done with work and log out of your session they will be deleted automatically.
Syntax for temporary table is :
CREATE TEMPORARY TABLE TABLE_NAME_HERE (key string, value string)
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.