要理解為什么MySQL查詢慢，首先要使用EXPLAIN語句分析查詢執(zhí)行計(jì)劃。1. EXPLAIN顯示查詢的執(zhí)行步驟，包括訪問的表、連接類型、索引使用情況等；2. 關(guān)鍵列如type（連接類型）、possible_keys和key（索引選擇）、rows（掃描行數(shù)）、Extra（額外信息）幫助識(shí)別性能瓶頸；3. 使用EXPLAIN時(shí)應(yīng)優(yōu)先檢查慢查詢?nèi)罩局械牟樵?，觀察是否有全表掃描（type: ALL）或高rows值；4. 注意Extra列中“Using filesort”或“Using temporary”等提示，可能意味著需要優(yōu)化排序或分組操作；5. 對(duì)于復(fù)雜查詢，可使用EXPLAIN FORMAT=JSON獲取更詳細(xì)的信息。通過這些步驟，可以有效識(shí)別并優(yōu)化缺失索引、錯(cuò)誤連接順序、不必要的排序等問題，從而提升查詢性能。

When you're trying to understand why a MySQL query is slow, one of the most powerful tools at your disposal is the EXPLAIN statement. It gives you insight into how MySQL executes your queries, helping you identify bottlenecks and optimize performance.

What EXPLAIN Shows You

Running EXPLAIN before a SELECT query returns information about how MySQL plans to execute it. This includes which tables are accessed, the join types used, whether indexes are effective, and more.

Key columns to pay attention to include:

• id: The identifier for the select query. Queries with multiple subqueries or unions will have multiple ids.
• select_type: Tells you if it's a simple SELECT, a subquery, or a UNION.
• table: Which table the row refers to.
• type: The join type—this is crucial. From best to worst: system, const, eq_ref, ref, range, index, ALL.
• possible_keys and key: Show which indexes MySQL considered and actually used.
• rows: Estimated number of rows MySQL must examine. Lower is better.
• Extra: Contains additional info like “Using filesort” or “Using temporary,” which are red flags.

Understanding these fields helps you make informed decisions about index usage and query structure.

How to Use EXPLAIN Effectively

To get the most out of EXPLAIN, follow these practices:

• Start with slow queries: Use the slow query log or tools like mysqldumpslow to find candidates.
• Run EXPLAIN on them: Add EXPLAIN before your SELECT and look for full table scans (type: ALL) or high rows values.
• Check the key usage: If possible_keys shows an index but key is empty, that index isn’t being used. Maybe your WHERE clause isn’t selective enough or uses functions on indexed columns.
• Watch the Extra column: Words like “Using filesort” or “Using temporary” suggest optimization opportunities.

For example, if you see a filesort, consider adding a covering index or reordering your ORDER BY.

You can also use EXPLAIN FORMAT=JSON for more detailed output, especially useful when analyzing complex joins or subqueries.

Common Optimization Scenarios

Here are a few real-world examples where EXPLAIN can guide you toward fixes:

• Missing Indexes:
  If type is ALL and possible_keys is NULL, you likely need an index. Try adding one on the relevant column(s) in the WHERE or JOIN clause.

• Incorrect Join Order:
  MySQL sometimes chooses a suboptimal order for joining tables. If you see large intermediate result sets early on, try restructuring the query or using STRAIGHT_JOIN.

• Unnecessary Sorting:
  If you see “Using filesort” in the Extra column, check if the sort is really needed. Sometimes adding an index that covers both the WHERE and ORDER BY clauses solves this.

• Temporary Tables in GROUP BY:
  If you see “Using temporary” along with “Using filesort” during a GROUP BY, consider optimizing the grouping columns or using derived tables.

Also, remember that EXPLAIN only shows the query plan—it doesn’t execute the query. So, it won’t reflect runtime issues like lock contention or disk I/O problems.

Final Notes

Using EXPLAIN regularly is a good habit when tuning queries. It’s not just for fixing slow queries—it’s also great for validating that your optimizations actually help. Sometimes adding an index doesn't change the plan as expected, and EXPLAIN lets you catch that early.

A common pitfall is assuming that any use of an index is good. That's not always true. An index scan (type: index) over many rows might be slower than a well-limited full table scan. Always check the rows value.

And don’t forget: EXPLAIN works only on SELECT statements. If you want to analyze UPDATE or DELETE queries, rewrite them as SELECTs first.

基本上就這些。

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