Gather statistics with advanced options (histograms, etc.)

Oracle

EXEC DBMS_STATS.GATHER_TABLE_STATS(
  ownname=>'HR', tabname=>'EMPLOYEE',
  estimate_percent=>DBMS_STATS.AUTO_SAMPLE_SIZE,
  method_opt=>'FOR ALL COLUMNS SIZE AUTO', degree=>4);

method_opt controls histograms and degree controls parallelism. Use LOCK_TABLE_STATS to lock statistics.

PostgreSQL

ALTER TABLE customer ALTER COLUMN city SET STATISTICS 500;
ANALYZE customer;

Per-column sampling target; default_statistics_target sets the global default.

MySQL

ANALYZE TABLE customer UPDATE HISTOGRAM ON city WITH 32 BUCKETS;

Histogram support in MySQL 8.0; useful on columns that are filtered on but not selective.

SQL Server

UPDATE STATISTICS dbo.customer WITH FULLSCAN;
-- Filtered statistics: CREATE STATISTICS ... WHERE ...

FULLSCAN is the most accurate; use SAMPLE on large tables. AUTO_UPDATE_STATISTICS for automatic updates.

MongoDB

// There are no manual statistics; the planner measures at runtime.
db.customer.getPlanCache().clear()

Apart from clearing a stale plan cache, no manual statistics are needed.

Redis

# Not applicable.

There are no statistics/optimizer.

Elasticsearch

# There is no statistics gathering; refresh searchability:
POST <index>/_refresh

Field data and doc values are used at query time; force merge reduces segments.