This error occurs when you try to use a reserved system column name (like tableoid, ctid, xmin, or xmax) as a regular column name in PostgreSQL. System columns are automatically added to every table and cannot be redefined.
PostgreSQL error 42939 (ERRCODE_RESERVED_NAME) is triggered when you attempt to create or reference a column using a name that PostgreSQL reserves for internal system columns. Every table in PostgreSQL automatically includes several implicit system columns with names like tableoid, ctid, xmin, xmax, cmin, and cmax. These columns are used by PostgreSQL to track transaction information, row versioning, and physical storage locations. When you try to define your own column with one of these reserved names, PostgreSQL rejects the operation to prevent conflicts with its internal metadata system. This is a syntax error that falls under SQLSTATE class 42 (Syntax Error or Access Rule Violation). Unlike SQL keywords that can be quoted to escape them, system column names are fundamentally reserved and cannot be used as user-defined column names even with double quotes. This is because these columns already exist implicitly in every table.
Review your CREATE TABLE or ALTER TABLE statement to find which column name is causing the conflict. Common reserved system column names include:
- tableoid: OID of the table containing this row
- ctid: Physical location of the row (tuple identifier)
- xmin: Transaction ID that inserted this row
- xmax: Transaction ID that deleted this row (0 if not deleted)
- cmin: Command ID within the inserting transaction
- cmax: Command ID within the deleting transaction
Check your error message or SQL statement for any of these names.
The only solution is to rename your column to something that doesn't conflict with system columns. Choose a descriptive alternative:
-- Instead of this (will fail):
CREATE TABLE my_table (
id SERIAL PRIMARY KEY,
ctid VARCHAR(50), -- Reserved system column name
data TEXT
);
-- Use this (will succeed):
CREATE TABLE my_table (
id SERIAL PRIMARY KEY,
correlation_id VARCHAR(50), -- Renamed to avoid conflict
data TEXT
);Common renaming patterns:
- tableoid → table_identifier or source_table_id
- ctid → correlation_id, tracking_id, or custom_id
- xmin → min_value or x_minimum
- xmax → max_value or x_maximum
If you're renaming a column in an existing schema, update all references in your application code:
-- If the table already exists, use ALTER TABLE:
ALTER TABLE my_table
RENAME COLUMN ctid TO correlation_id;Then update your application queries:
// Before (with reserved name):
const query = 'SELECT id, ctid, data FROM my_table';
// After (with new name):
const query = 'SELECT id, correlation_id, data FROM my_table';Search your codebase for all references to the old column name and update them.
Before renaming, confirm you're not trying to reinvent functionality that system columns already provide:
-- You can query system columns directly (they're always available):
SELECT
id,
data,
tableoid::regclass AS table_name, -- Which table this row belongs to
ctid, -- Physical row location
xmin, -- Transaction that created this row
xmax -- Transaction that deleted this row
FROM my_table;If you need transaction tracking or row versioning, consider using the built-in system columns instead of creating your own. They're hidden from SELECT * but can be accessed explicitly when needed.
Understanding PostgreSQL System Columns:
System columns are fundamental to PostgreSQL's MVCC (Multi-Version Concurrency Control) implementation. They enable features like:
- Row versioning: xmin and xmax track which transactions can see each row version
- Physical storage: ctid identifies the exact disk location (page and offset)
- Inheritance queries: tableoid identifies which table in an inheritance hierarchy contains a row
- Vacuuming: PostgreSQL uses these columns to determine when row versions can be removed
Why These Names Cannot Be Quoted:
Unlike SQL keywords, system column names cannot be escaped with double quotes because they're not just reserved words—they're actual columns that exist in every table. When you execute SELECT * FROM my_table, PostgreSQL deliberately hides these columns from the output, but they're still there.
Migration Considerations:
If you're migrating from MySQL, Oracle, or SQL Server, be aware that these databases don't have the same system column restrictions. You may need to scan your schemas for columns named xmin, xmax, ctid, etc., and rename them before migration.
ORM and Framework Implications:
Some ORMs (like Django, SQLAlchemy, or Prisma) may generate column names automatically. If they conflict with system columns, you'll need to explicitly override the column naming in your model definitions:
# Django example - override auto-generated column name:
class MyModel(models.Model):
custom_tracking_id = models.CharField(max_length=50, db_column='tracking_id')Performance Note:
While system columns like ctid can be used for quick row lookups, be aware that ctid values change when rows are updated or when VACUUM FULL runs. Never use ctid as a permanent row identifier—use proper PRIMARY KEY constraints instead.
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