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.brain/.agent/skills/database-optimization/mysql/references/index-maintenance.md

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+---
+title: Index Maintenance and Cleanup
+description: Index maintenance
+tags: mysql, indexes, maintenance, unused-indexes, performance
+---
+
+# Index Maintenance
+
+## Find Unused Indexes
+
+```sql
+-- Requires performance_schema enabled (default in MySQL 5.7+)
+-- "Unused" here means no reads/writes since last restart.
+SELECT object_schema, object_name, index_name, COUNT_READ, COUNT_WRITE
+FROM performance_schema.table_io_waits_summary_by_index_usage
+WHERE object_schema = 'mydb'
+  AND index_name IS NOT NULL AND index_name != 'PRIMARY'
+  AND COUNT_READ = 0 AND COUNT_WRITE = 0
+ORDER BY COUNT_WRITE DESC;
+```
+
+Sometimes you'll also see indexes with **writes but no reads** (overhead without query benefit). Review these carefully: some are required for constraints (UNIQUE/PK) even if not used in query plans.
+
+```sql
+SELECT object_schema, object_name, index_name, COUNT_READ, COUNT_WRITE
+FROM performance_schema.table_io_waits_summary_by_index_usage
+WHERE object_schema = 'mydb'
+  AND index_name IS NOT NULL AND index_name != 'PRIMARY'
+  AND COUNT_READ = 0 AND COUNT_WRITE > 0
+ORDER BY COUNT_WRITE DESC;
+```
+
+Counters reset on restart — ensure 1+ full business cycle of uptime before dropping.
+
+## Find Redundant Indexes
+
+Index on `(a)` is redundant if `(a, b)` exists (leftmost prefix covers it). Pairs sharing only the first column (e.g. `(a,b)` vs `(a,c)`) need manual review — neither is redundant.
+
+```sql
+-- Prefer sys schema view (MySQL 5.7.7+)
+SELECT table_schema, table_name,
+  redundant_index_name, redundant_index_columns,
+  dominant_index_name, dominant_index_columns
+FROM sys.schema_redundant_indexes
+WHERE table_schema = 'mydb';
+```
+
+## Check Index Sizes
+
+```sql
+SELECT database_name, table_name, index_name,
+  ROUND(stat_value * @@innodb_page_size / 1024 / 1024, 2) AS size_mb
+FROM mysql.innodb_index_stats
+WHERE stat_name = 'size' AND database_name = 'mydb'
+ORDER BY stat_value DESC;
+-- stat_value is in pages; multiply by innodb_page_size for bytes
+```
+
+## Index Write Overhead
+Each index must be updated on INSERT, UPDATE, and DELETE operations. More indexes = slower writes.
+
+- **INSERT**: each secondary index adds a write
+- **UPDATE**: changing indexed columns updates all affected indexes
+- **DELETE**: removes entries from all indexes
+
+InnoDB can defer some secondary index updates via the change buffer, but excessive indexing still reduces write throughput.
+
+## Update Statistics (ANALYZE TABLE)
+The optimizer relies on index cardinality and distribution statistics. After large data changes, refresh statistics:
+
+```sql
+ANALYZE TABLE orders;
+```
+
+This updates statistics (does not rebuild the table).
+
+## Rebuild / Reclaim Space (OPTIMIZE TABLE)
+`OPTIMIZE TABLE` can reclaim space and rebuild indexes:
+
+```sql
+OPTIMIZE TABLE orders;
+```
+
+For InnoDB this effectively rebuilds the table and indexes and can be slow on large tables.
+
+## Invisible Indexes (MySQL 8.0+)
+Test removing an index without dropping it:
+
+```sql
+ALTER TABLE orders ALTER INDEX idx_status INVISIBLE;
+ALTER TABLE orders ALTER INDEX idx_status VISIBLE;
+```
+
+Invisible indexes are still maintained on writes (overhead remains), but the optimizer won't consider them.
+
+## Index Maintenance Tools
+
+### Online DDL (Built-in)
+Most add/drop index operations are online-ish but still take brief metadata locks:
+
+```sql
+ALTER TABLE orders ADD INDEX idx_status (status), ALGORITHM=INPLACE, LOCK=NONE;
+```
+
+### pt-online-schema-change / gh-ost
+For very large tables or high-write workloads, online schema change tools can reduce blocking by using a shadow table and a controlled cutover (tradeoffs: operational complexity, privileges, triggers/binlog requirements).
+
+## Guidelines
+- 1–5 indexes per table is normal. 6+: audit for redundancy.
+- Combine `performance_schema` data with `EXPLAIN` of frequent queries monthly.