MySQL/PostgreSQL是否缓存查询解析编译？条件顺序影响性能吗？

Let's break down your questions clearly, as someone who's worked with both databases extensively:

Do databases re-parse/compile identical queries every time? #

Neither MySQL nor PostgreSQL will waste resources re-parsing and compiling an identical query on every execution—both have built-in mechanisms to avoid redundant work, though the details vary:

• MySQL: The old result-set query cache was removed in MySQL 8.0 (due to low hit rates and high maintenance overhead), but it still caches execution plans. If your SQL text is exactly identical (including whitespace, comments, and clause order), it will reuse the pre-generated plan instead of redoing parsing and optimization.
• PostgreSQL: It relies on a Plan Cache to store fully optimized execution plans. When an identical SQL statement is submitted, it skips parsing, rewriting, and optimization steps entirely by reusing the cached plan.

Will queries with different condition orders be treated differently? #

Looking at your two example queries:

/* first query */ SELECT * FROM USER WHERE age = 20 AND name = 'foo';
/* second query */ SELECT * FROM USER WHERE name = 'foo' AND age = 20;

These are considered distinct statements by both databases because their raw text differs (the comments are unique, and the order of AND conditions is reversed). That means the database will parse, compile, and cache separate plans for each.

But here's the critical point: even though the plans are cached separately, the actual execution performance will be identical. The query optimizer in both databases ignores the order of AND conditions—it analyzes table statistics (like index presence, column cardinality, and data distribution) to choose the most efficient execution path, regardless of how you order conditions in your SQL.

Even if you stripped out the comments and just reversed the conditions, the SQL text would still be different, so the databases wouldn't reuse the cached plan. But again, the optimizer would generate an equivalent optimal plan for both queries, so there's no performance hit during execution itself.

Implications for your SQL generation tool #

While execution performance isn't affected, generating lots of similar queries with just condition order reversed can bloat the plan cache with redundant, equivalent plans, wasting memory over time.

To address this:

• Standardize condition order: For example, sort your WHERE clause conditions alphabetically by column name. This ensures the generated SQL text is identical every time for the same set of conditions, allowing full plan reuse.
• Use parameterized queries: Instead of hardcoding values like 20 or 'foo', use placeholders (e.g., SELECT * FROM USER WHERE age = ? AND name = ?). Parameterized queries let the database reuse the same plan regardless of parameter values or condition order (as long as the query structure is identical), and they also protect against SQL injection.

内容的提问来源于stack exchange，提问作者mux