Working with Identifiers and Schemas in PostgreSQL

PostgreSQL is a powerful, open-source relational database system. A common issue beginners (and sometimes experienced developers) encounter is the "relation does not exist" error when querying tables. This tutorial explains how PostgreSQL handles identifiers (table and column names) and schemas, and how to avoid this error.

Understanding Identifiers

In PostgreSQL, identifiers are names used for database objects like tables, columns, and functions. Here’s what you need to know:

• Case Sensitivity: PostgreSQL is case-sensitive by default. This means myTable and MyTable are treated as distinct identifiers. However, PostgreSQL automatically converts unquoted identifiers to lowercase. This can lead to confusion.
• Quoting Identifiers: To preserve case or use special characters (like spaces or punctuation) in identifiers, you must enclose them in double quotes ("). For example, "MyTable" or "Table with spaces".
• Unquoted Identifiers: If you don’t use double quotes, PostgreSQL folds the identifier to lowercase. This means SELECT * FROM MyTable; is equivalent to SELECT * FROM mytable;.

Example:

-- Create a table with mixed-case name (requires quoting)
CREATE TABLE "MyTable";

-- Attempting to query without quotes (fails if the table was created with mixed case)
-- SELECT * FROM MyTable; -- This will likely fail

-- Correct way to query
SELECT * FROM "MyTable";

-- Create another table with all lowercase
CREATE TABLE mytable2;

-- Can query this without quotes (since it's all lowercase)
SELECT * FROM mytable2;

Working with Schemas

A schema is a namespace within a database that organizes database objects. It’s like a folder system for your tables, views, functions, and other database components. The default schema is public.

Why use schemas?

• Organization: Schemas help you logically group related tables and objects.
• Security: You can grant different permissions to different schemas.
• Name Conflicts: Schemas prevent naming conflicts between objects in different parts of your database.

Querying Tables in Different Schemas

If a table resides in a schema other than the current search path (explained below), you need to qualify the table name with the schema name.

Example:

-- Create a table in a schema named "myschema"
CREATE SCHEMA myschema;
CREATE TABLE myschema.my_table;

-- Query the table (must include the schema name)
SELECT * FROM myschema.my_table;

The search_path

The search_path is a list of schemas that PostgreSQL searches when you reference a table name without specifying the schema. When you execute a query like SELECT * FROM mytable;, PostgreSQL searches the schemas in the search_path in order until it finds a table named mytable.

Checking the search_path

You can view the current search_path using:

SHOW search_path;

The output typically looks like "$user",public. This means PostgreSQL first searches the schema owned by the current user, then the public schema.

Setting the search_path

You can modify the search_path using the SET command:

SET search_path TO myschema, public;

This tells PostgreSQL to first search myschema, then public. After setting the search_path, you can query tables in myschema without qualifying them with the schema name. Note that this change is only for the current session.

Example:

-- Set the search path
SET search_path TO myschema, public;

-- Now you can query tables in myschema without specifying the schema
SELECT * FROM mytable; -- If mytable exists in myschema

Best Practices

• Consistent Naming: Choose a consistent naming convention (e.g., all lowercase or camel case) for your tables and columns.
• Avoid Uppercase: While PostgreSQL allows uppercase names with quoting, it’s generally best to avoid them to simplify queries and improve readability.
• Explicit Schema Qualification: For clarity and to avoid ambiguity, especially in larger projects, always qualify table names with the schema name, even if the schema is in the search_path.
• Understand the search_path: Be aware of how the search_path affects your queries and adjust it as needed.