A covering index contains the columns needed to evaluate a particular query and return its requested values. In InnoDB, secondary-index records also carry the primary-key columns, so a query can be covered without explicitly listing the primary key again in the secondary-index definition. Coverage is a relationship between one query and an index, not a permanent property that applies to every query against the table. ...
Partition pruning excludes entire partitions that cannot contain matching rows. It is different from using an index to find rows within a remaining partition, and it is not a page-level cache trick. PostgreSQL can perform this exclusion while planning a query or later when parameter values become available during execution. ...
A common table expression gives a query result a name within one SQL statement. It can make a complex report readable by naming populations and intermediate calculations before the final SELECT. That name is not a persistent table, and it does not by itself promise a cached result, a particular scan count, or a sequence of procedural steps. ...
EXPLAIN shows the execution plan selected for a statement. It helps locate expensive work and mistaken estimates, but a slow request can also involve locks, network transfer, or application behavior outside the plan. Use plan evidence to test a specific hypothesis rather than assuming that every performance problem requires a different scan or another index. ...