What is a Deadlock? A deadlock occurs when two or more transactions are waiting for each other to release locks, and neither can proceed. Oracle automatically detects deadlocks and kills one transaction, but that’s a last resort. Example of a Deadlock: They’re both stuck — deadlock. Best Practices to Avoid Deadlocks in Oracle 1. Always…

Database locks are essential mechanisms that ensure data integrity and consistency by controlling concurrent access. Without proper locking, multiple transactions might interfere with each other, leading to issues like dirty reads, lost updates, and inconsistencies. Locking mechanisms help coordinate database operations in multi-user environments, preventing conflicts and ensuring data correctness. Here, we will explore the…

Introduction Isolation is a key property of database transactions, ensuring that concurrent transactions do not interfere with each other and maintain data consistency. Databases enforce isolation using different concurrency control mechanisms, including: Each method has its trade-offs between consistency, performance, and concurrency. Let’s explore them in detail. 1. Locking – Pessimistic Concurrency Control Definition Locking…

Introduction Isolation levels define how transactions interact with each other in a database. The goal is to balance consistency, performance, and concurrency. SQL databases provide four standard isolation levels: Each level addresses different types of concurrency problems, such as dirty reads, non-repeatable reads, and phantom reads. 1. Read Uncommitted (Lowest Isolation Level) Definition: At this…

Introduction Concurrency control is crucial in database management systems (DBMS) to ensure data consistency and integrity when multiple transactions execute simultaneously. However, improper handling of concurrent transactions can lead to anomalies such as Dirty Read, Non-Repeatable Read, and Phantom Read. Understanding these anomalies helps in choosing the appropriate isolation level to maintain data consistency. 1.…

1. Overview The EXISTS condition in Oracle is a powerful operator used in SQL queries to check whether a subquery returns any rows. If the subquery returns at least one row, the EXISTS condition evaluates to TRUE; otherwise, it evaluates to FALSE. This feature is commonly used in scenarios requiring existence checks or correlated subqueries.…

1. Overview An anti-join in Oracle is used to find rows in one table that do not have matching rows in another table. It is essentially the opposite of a semi-join. Anti-joins are often implemented using the NOT EXISTS, NOT IN, or LEFT JOIN … IS NULL constructs. Anti-joins are particularly useful for identifying unmatched…

1. Overview A semi-join in Oracle is a type of query where rows from one table are returned if they satisfy a condition with at least one row from another table. Unlike regular joins, a semi-join only returns rows from the first table, even if multiple matching rows exist in the second table. It is…

1. Overview An outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition. An OUTER JOIN in Oracle retrieves rows from two…

1. Overview An inner join (sometimes called a simple join) is a join of two or more tables that returns only those rows that satisfy the join condition. An equi join is a subset of inner join that specifically uses the equality operator (=) in the join condition. In Oracle, an INNER JOIN is used…