How Do WHERE Clauses Impact the Performance of JOINs and LEFT JOINs in PostgreSQL?

JOIN vs. LEFT JOIN: Performance Implications of WHERE Clause Usage

In database queries, the JOIN clause is used to combine rows from multiple tables based on a common column. The LEFT JOIN variation retains all rows from the left table, even if there are no matching rows in the right table. A question has been raised regarding the relative performance of JOIN and LEFT JOIN when used in conjunction with WHERE clauses.

JOIN vs. WHERE Clause Performance

Contrary to common assumptions, WHERE conditions and JOIN conditions are generally equivalent for INNER JOINs in PostgreSQL. Explicit JOIN conditions are considered good practice for readability and maintainability.

However, the combination of LEFT JOIN and WHERE conditions becomes crucial. LEFT JOIN preserves all rows from the left table, even with no matches in the right table. If a WHERE condition subsequently filters out null values from the right table, LEFT JOIN behaves like an INNER JOIN but potentially with a less optimal query plan.

Impact on Query Optimization

In complex queries involving multiple joined tables, identifying the optimal join sequence is computationally costly. The "Generic Query Optimizer" seeks to find the best query plan. Misleading use of LEFT JOIN can complicate the optimizer's task and lead to performance issues.

Related Issues

Additional resources and examples demonstrate the potential problems that can arise from incorrect WHERE clause usage with LEFT JOIN:

• [What does [FROM x, y] mean in Postgres?](https://dba.stackexchange.com/questions/261291/what-does-from-x-y-mean-in-postgres)
• ["invalid reference to FROM-clause entry for table" in Postgres query](https://stackoverflow.com/questions/1533937/invalid-reference-to-from-clause-entry-for-table-in-postgres-query)
• [Why does null seem to equal an integer in WHERE?](https://stackoverflow.com/questions/8232136/why-does-null-seem-to-equal-an-integer-in-where)
• [Left outer join acting like inner join](https://stackoverflow.com/questions/63092044/left-outer-join-acting-like-inner-join)

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