Let's say I am writing a query template to query any table in a PostgreSQL 14 database. The template takes 3 inputs:
table_name, page_number, rows_per_page
And looks like this:
SELECT *
FROM <table_name>
LIMIT <rows_per_page> OFFSET (<page_number> -1)*<rows_per_page>
The problem with the above 'pagination' is that it is unreliable, Postgres doc says that without ORDER BY clause will produce inconsistent results. My question is: is there a way to remedy this problem for a generic table i.e. a table that has no known way to order it (no primary key)? Basically what I am asking is: is there any 'hidden' ordering that Postgres is using and I can access through a query or the tables that have no primary key are inherently unordered?
Related
I can use the pg_indexes view to get the SQL used to create an index (as in the sql in this question: How to get indexes, primary keys, and all constraints for a schema in PostgreSQL using standard sql).
Is there a way to get the SQL for constraints in PostgreSQL? I'm not seeing a pg_constraints table and the information I'm finding on line isn't pointing me to a solution, for example, these are some of the hits coming up when I Google the terms (postgresql get sql for constraint).
https://dba.stackexchange.com/questions/206562/postgres-read-constraints-definition
Postgres Check Constraint definition
https://www.geeksforgeeks.org/sql-query-to-display-all-the-existing-constraints-on-a-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.
Recently I saw a strange scenario with my PostgreSQL DB. The information schema of my database is showing a different sequence name than the one actually allocated for the column of my table.
The issue is:
I have a table tab_1
id name
1 emp1
2 emp2
3 emp3
Previously the id column (integer) of the table was an auto generated field where the sequence number was generated at run time via JPA. (Sequence name: tab_1_seq)
We made a change and updated the table's column id to bigserial and the sequence is maintained in the column level (allocated new sequence: tab_1_temp_seq) not handled by the JPA anymore.
After this change everything was working fine for few months and after that we faced an error - "the sequence "tab_1_temp_seq" is not yet defined in this session"
On analyzing the issue I found out that there is a mismatch between the sequences allocated for the table.
In the table structure, we where shown the sequence as tab_1_temp_seq and in the information_schema the table was allocated with the old sequence - tab_1_seq.
I am not sure what has really triggered this to happen, as we are not managing our database system. If you have faced any issues like this, kindly let me know its root cause.
Queries:
SELECT table_name, column_name, column_default from information_schema.columns where table_name = ‘tab_1’;
result :
table_name column_name column_default
tab_1 id nextval('tab_1_seq::regclass')
Below are the details found in the table structure/properties:
id nextval('tab_1_temp_seq::regclass')
name varChar
Perhaps you are suffering from data corruption, but it is most likely that you are suffering from bad tools to visualize your database objects. Whatever program shows you the “table structure/properties” might be confused.
To find out the truth (which DEFAULT value PostgreSQL uses), run:
SELECT pg_get_expr(adbin, adrelid)
FROM pg_attrdef
WHERE adrelid = 'tab1'::regclass;
This is also what information_schema.columns will show, but I added the naked query for clarity.
This DEFAULT value will be used whenever the INSERT statement either doesn't specify the id column or fills it with the special value DEFAULT.
Perhaps the confusion is also caused by different programs that may set default values in their way. The way I showed above is PostgreSQL's way, but nothing can keep a third-party tool from using its own sequence to filling the id.
Following the blog of Rob Conery I have set of unique IDs across the tables of my Postgres DB.
Now, using these unique IDs, is there a way to query a row on the DB without knowing what table it is in? Or can those tables be indexed such that if the row is not available on the current table, I just increase the index and I can query to the next table?
In short - if you did not prepared for that - then no. You can prepare for that by generating your own uuid. Please look here. For instance PG has uuid that preserve order. Also uuid v5 has something like namespaces. So you can build hierarchy. However that is done by hashing namespace, and I don't know tool to do opposite inside PG.
If you know all possible tables in advance you could prepare a query that simply UNIONs a search with a tagged type over all tables. In case of two tables named comments and news you could do something like:
PREPARE type_of_id(uuid) AS
SELECT id, 'comments' AS type
FROM comments
WHERE id = $1
UNION
SELECT id, 'news' AS type
FROM news
WHERE id = $1;
EXECUTE type_of_id('8ecf6bb1-02d1-4c04-8875-f1da62b7f720');
Automatically generating this could probably be done by querying pg_catalog.pg_tables and generating the relevant query on the fly.
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.