Migrating from MySQL to PostgreSQL Key Query Differences and Considerations

Switching from MySQL to PostgreSQL requires careful consideration of query syntax, data types, and database features. This guide highlights key distinctions to facilitate a smooth migration process.

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1. Data Type Mapping:

MySQL and PostgreSQL employ different data types. Here's a comparison:

MySQL Data Type	PostgreSQL Equivalent	Notes
TINYINT	SMALLINT	Use BOOLEAN for true/false.
DATETIME	TIMESTAMP	Consider TIMESTAMPTZ for timezone awareness.
TEXT	TEXT	Functionally identical.
ENUM	TEXT CHECK constraint	PostgreSQL lacks ENUM; simulate using CHECK constraints.
AUTO_INCREMENT	SERIAL or GENERATED AS IDENTITY	Use SERIAL or GENERATED AS IDENTITY for auto-incrementing keys.
DOUBLE	DOUBLE PRECISION	Direct equivalent.
BLOB	BYTEA	For binary large objects.

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2. Case Sensitivity in String Comparisons:

Case Sensitivity Differences:

• MySQL: LIKE is case-insensitive by default (for non-binary columns).
• PostgreSQL: LIKE is case-sensitive; use ILIKE for case-insensitive matching.

Wildcards: Both databases use % (zero or more characters) and _ (single character) wildcards.

Examples:

• MySQL (case-insensitive): SELECT * FROM users WHERE name LIKE 'john%';
• PostgreSQL (case-insensitive equivalent): SELECT * FROM users WHERE name ILIKE 'john%'; or SELECT * FROM users WHERE LOWER(name) LIKE 'john%';

Optimization: For efficient case-insensitive searches in PostgreSQL, create a functional index: CREATE INDEX idx_users_name_lower ON users (LOWER(name));

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3. Auto-Incrementing Primary Keys:

• MySQL: Uses AUTO_INCREMENT.
• PostgreSQL: Employs SERIAL or GENERATED AS IDENTITY.

Examples:

• MySQL: CREATE TABLE users (id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(100));
• PostgreSQL: CREATE TABLE users (id SERIAL PRIMARY KEY, name VARCHAR(100));

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4. String Functions:

PostgreSQL's string function syntax may differ slightly from MySQL's.

MySQL Function	PostgreSQL Equivalent
CONCAT()	CONCAT()
LENGTH()	LENGTH()
SUBSTRING()	SUBSTRING()
LOCATE()	POSITION() or STRPOS()
REPLACE()	REPLACE()

Example: Both databases use CONCAT() identically: SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;

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5. Pagination (LIMIT and OFFSET):

Both databases support LIMIT and OFFSET with identical syntax: SELECT * FROM users LIMIT 10 OFFSET 20;

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6. Default Values:

PostgreSQL's default value handling is stricter.

• MySQL: CREATE TABLE orders (id INT AUTO_INCREMENT PRIMARY KEY, created_at DATETIME DEFAULT CURRENT_TIMESTAMP);
• PostgreSQL: CREATE TABLE orders (id SERIAL PRIMARY KEY, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP);

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7. Joins:

PostgreSQL enforces stricter SQL join standards. Ensure join columns have compatible data types or use explicit casting: SELECT * FROM orders JOIN customers ON orders.customer_id = customers.id::TEXT;

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8. Full-Text Search:

• MySQL: Uses FULLTEXT indexing.
• PostgreSQL: Leverages TSVECTOR and TSQUERY for advanced full-text search.

Examples:

• MySQL: SELECT * FROM articles WHERE MATCH(content) AGAINST('search term');
• PostgreSQL: SELECT * FROM articles WHERE content @@ to_tsquery('search & term');

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9. Foreign Key Constraints:

PostgreSQL enforces more rigorous foreign key constraints. Verify schema and data integrity.

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10. Case Sensitivity of Identifiers:

• MySQL: Table and column names are generally case-insensitive (unless using a binary collation).
• PostgreSQL: Table and column names are case-sensitive when quoted ("table_name").

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11. Stored Procedures:

Stored procedure syntax differs substantially.

• MySQL: Uses DELIMITER to define procedures.
• PostgreSQL: Uses DO blocks or CREATE FUNCTION.

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12. Indexes:

PostgreSQL provides advanced indexing options (GIN, GiST, BRIN) and supports functional indexes.

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Migration Strategy:

1. Utilize migration tools like pgLoader or AWS DMS for automated schema and data transfer.
2. Manually review and adjust SQL queries, particularly those involving case sensitivity, auto-increment, full-text search, joins, and stored procedures.
3. Optimize indexes for PostgreSQL's capabilities.
4. Conduct thorough testing in a staging environment before production migration.

A thorough understanding of these differences ensures a successful and efficient migration from MySQL to PostgreSQL.

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