This PostgreSQL error occurs when you attempt to create a prepared statement with a name that already exists in the current session. The solution is to deallocate the previous statement or use a different name.
In PostgreSQL, prepared statements are named server-side objects that persist for the duration of a database session. Each prepared statement name must be unique within a single session. When PostgreSQL encounters an attempt to create a prepared statement with a name that already exists—without first deallocating the previous statement—it raises error code 42P05 (duplicate_prepared_statement). This typically happens in application code where prepared statements are created dynamically or in loops without proper cleanup. The error can also occur when reusing connection pooled connections that retain prepared statements from previous transactions.
If you control the prepared statement creation, add a DEALLOCATE statement before re-preparing with the same name. This removes the existing statement and allows you to create a new one with that name.
DEALLOCATE PREPARE statement_name;
PREPARE statement_name AS SELECT * FROM table WHERE id = $1;Alternatively, use the DEALLOCATE ALL command to clear all prepared statements:
DEALLOCATE ALL;PostgreSQL allows unnamed prepared statements (empty string as name). When you use an unnamed statement, any previous unnamed statement is automatically replaced without error.
-- This replaces any previous unnamed statement automatically
PREPARE AS SELECT * FROM users WHERE id = $1;
EXECUTE (1);
DEALLOCATE;This approach avoids the naming issue entirely for simple use cases.
Java applications using PostgreSQL JDBC drivers sometimes see this error due to the driver's automatic prepared statement caching. Disable automatic prepared statement caching by adding the prepareThreshold parameter to your connection string:
jdbc:postgresql://localhost:5432/mydb?prepareThreshold=0This parameter tells the driver not to automatically convert queries to prepared statements, eliminating the naming collision issue. Note this may have performance implications for query-heavy applications.
When using PgBouncer (a PostgreSQL connection pooler), prepared statements created in one client connection may collide if requests route to different database connections.
Option 1: Switch PgBouncer to 'session' pooling mode instead of 'transaction' mode in pgbouncer.ini:
pool_mode = sessionOption 2: Disable prepared statement caching in your application when using PgBouncer.
Session pooling ensures the same client always uses the same backend connection, preserving prepared statement state.
If you must create multiple prepared statements with similar purposes, generate unique names for each:
-- Using timestamp
PREPARE stmt_<timestamp> AS SELECT * FROM users WHERE id = $1;
-- Using application ID or session identifier
PREPARE stmt_<app_id>_<query_hash> AS SELECT * FROM users WHERE id = $1;This approach prevents name collisions while still allowing you to manage lifecycle of multiple statements.
Name Truncation Issue: PostgreSQL truncates prepared statement names to 63 characters (NAMEDATALEN). If two different prepared statements have the same first 63 characters, they will collide. Ensure your naming scheme produces names shorter than 63 characters.
Statement Persistence Across Transactions: Unlike some databases, PostgreSQL prepared statements persist across transaction rollbacks within the same session. If a transaction fails after creating a prepared statement, you must still deallocate it before reusing the name.
Connection Pooling Considerations: Different pooling modes (transaction vs session) have different implications for prepared statements. Transaction pooling can route requests to different backend connections, so prepared statements from one transaction may not be available in the next. Understanding your pooling strategy is crucial for applications that rely on prepared statements.
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