PostgreSQL Interview Questions and Answers for experienced

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100 PostgreSQL Interview Questions and Answers

  1. What are the different data types supported by PostgreSQL?

    • Answer: PostgreSQL supports a wide range of data types including integers (INT, BIGINT, SMALLINT), floating-point numbers (FLOAT, REAL, DOUBLE PRECISION), numeric (NUMERIC, DECIMAL), character strings (VARCHAR, TEXT, CHAR), boolean (BOOLEAN), dates and times (DATE, TIME, TIMESTAMP, INTERVAL), binary data (BYTEA), arrays, JSON, JSONB, UUIDs, and many more. The choice of data type depends on the specific needs of the application.

  2. Explain the difference between `INNER JOIN`, `LEFT JOIN`, `RIGHT JOIN`, and `FULL OUTER JOIN`.

    • Answer: These are all join types used to combine rows from two or more tables based on a related column. `INNER JOIN` returns only the rows where the join condition is met in both tables. `LEFT JOIN` returns all rows from the left table, even if there is no match in the right table (NULLs for unmatched columns in the right table). `RIGHT JOIN` is the opposite, returning all rows from the right table. `FULL OUTER JOIN` returns all rows from both tables; if a row has a match in the other table, the corresponding columns are populated; otherwise, NULLs are used for unmatched columns.

  3. What are indexes and why are they important in PostgreSQL?

    • Answer: Indexes are special lookup tables that the database search engine can use to speed up data retrieval. Simply put, an index in PostgreSQL is a pointer to data in a table. They significantly improve query performance by allowing the database to quickly locate rows without scanning the entire table. Different index types exist, including B-tree (default for most cases), GiST, GIN, BRIN, and SP-GiST, each optimized for different data types and query patterns.

  4. How do you handle transactions in PostgreSQL?

    • Answer: Transactions in PostgreSQL ensure data integrity and consistency. They are managed using commands like `BEGIN`, `COMMIT`, and `ROLLBACK`. `BEGIN` starts a transaction, `COMMIT` saves changes permanently, and `ROLLBACK` undoes all changes made within the transaction. Transactions offer ACID properties (Atomicity, Consistency, Isolation, Durability).

  5. Explain the concept of stored procedures in PostgreSQL.

    • Answer: Stored procedures are pre-compiled SQL code blocks that can be stored and reused within the database. They improve performance by reducing the need for repeated parsing and compilation of SQL statements. They can accept parameters, perform complex operations, and return results. They enhance code reusability and maintainability.

  6. What are views in PostgreSQL and how are they used?

    • Answer: Views are virtual tables based on the result-set of an SQL statement. They don't store data themselves; instead, they provide a customized way to access data from one or more underlying tables. Views simplify complex queries, provide data security by restricting access to specific columns, and improve data abstraction.

  7. Describe different types of PostgreSQL constraints.

    • Answer: PostgreSQL supports various constraints to enforce data integrity: `NOT NULL` (ensures a column cannot have NULL values), `UNIQUE` (prevents duplicate values in a column or a set of columns), `PRIMARY KEY` (uniquely identifies each row in a table), `FOREIGN KEY` (establishes a link between two tables), `CHECK` (ensures that values in a column satisfy a specific condition), and more.

  8. How do you handle NULL values in PostgreSQL?

    • Answer: NULL represents the absence of a value. Use `IS NULL` and `IS NOT NULL` in WHERE clauses to check for NULL values. Functions like `COALESCE` (returns the first non-NULL value) and `NULLIF` (returns NULL if two values are equal) are helpful in handling NULLs. The `NVL` function (available through extensions) provides similar functionality to COALESCE.

  9. Explain the concept of database triggers in PostgreSQL.

    • Answer: Triggers are stored procedures automatically executed in response to certain events on a particular table or view. These events can be INSERT, UPDATE, or DELETE operations. Triggers are useful for implementing complex business rules, auditing data changes, enforcing constraints, and maintaining data consistency.

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