F.29. pg_overexplain — allow EXPLAIN to dump even more details
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## F.29. pg_overexplain — allow EXPLAIN to dump even more details #

The `pg_overexplain` module extends `EXPLAIN` with new options that provide additional output. It is mostly intended to assist with debugging of and development of the planner, rather than for general use. Since this module displays internal details of planner data structures, it may be necessary to refer to the source code to make sense of the output. Furthermore, the output is likely to change whenever (and as often as) those data structures change.

To use it, simply load it into the server. You can load it into an individual session:

    LOAD 'pg_overexplain';

You can also preload it into some or all sessions by including `pg_overexplain` in session_preload_libraries or shared_preload_libraries in `postgresql.conf`.

### F.29.1. EXPLAIN (DEBUG) #

The `DEBUG` option displays miscellaneous information from the plan tree that is not normally shown because it is not expected to be of general interest. For each individual plan node, it will display the following fields. See `Plan` in `nodes/plannodes.h` for additional documentation of these fields.

- `Disabled Nodes`. Normal `EXPLAIN` determines whether a node is disabled by checking whether the node's count of disabled nodes is larger than the sum of the counts for the underlying nodes. This option shows the raw counter value.

- `Parallel Safe`. Indicates whether it would be safe for a plan tree node to appear beneath a `Gather` or `Gather Merge` node, regardless of whether it is actually below such a node.

- `Plan Node ID`. An internal ID number that should be unique for every node in the plan tree. It is used to coordinate parallel query activity.

- `extParam` and `allParam`. Information about which numbered parameters affect this plan node or its children. In text mode, these fields are only displayed if they are non-empty sets.

Once per query, the `DEBUG` option will display the following fields. See `PlannedStmt` in `nodes/plannodes.h` for additional detail.

- `Command Type`. For example, `select` or `update`.

- `Flags`. A comma-separated list of Boolean structure member names from the `PlannedStmt` that are set to `true`. It covers the following structure members: `hasReturning`, `hasModifyingCTE`, `canSetTag`, `transientPlan`, `dependsOnRole`, `parallelModeNeeded`.

- `Subplans Needing Rewind`. Integer IDs of subplans that may need to be rewound by the executor.

- `Relation OIDs`. OIDs of relations upon which this plan depends.

- `Executor Parameter Types`. Type OID for each executor parameter (e.g. when a nested loop is chosen and a parameter is used to pass a value down to an inner index scan). Does not include parameters supplied to a prepared statement by the user.

- `Parse Location`. Location within the query string supplied to the planner where this query's text can be found. May be `Unknown` in some contexts. Otherwise, may be `NNN to end` for some integer `NNN` or `NNN for MMM bytes` for some integers `NNN` and `MMM`.

### F.29.2. EXPLAIN (RANGE_TABLE) #

The `RANGE_TABLE` option displays information from the plan tree specifically concerning the query's range table. Range table entries correspond roughly to items appearing in the query's `FROM` clause, but with numerous exceptions. For example, subqueries that are proved unnecessary may be deleted from the range table entirely, while inheritance expansion adds range table entries for child tables that are not named directly in the query.

Range table entries are generally referenced within the query plan by a range table index, or RTI. Plan nodes that reference one or more RTIs will be labelled accordingly, using one of the following fields: `Scan RTI`, `Nominal RTI`, `Exclude Relation RTI`, `Append RTIs`.

In addition, the query as a whole may maintain lists of range table indexes that are needed for various purposes. These lists will be displayed once per query, labelled as appropriate as `Unprunable RTIs` or `Result RTIs`. In text mode, these fields are only displayed if they are non-empty sets.

Finally, but most importantly, the `RANGE_TABLE` option will display a dump of the query's entire range table. Each range table entry is labelled with the appropriate range table index, the kind of range table entry (e.g. `relation`, `subquery`, or `join`), followed by the contents of various range table entry fields that are not normally part of `EXPLAIN` output. Some of these fields are only displayed for certain kinds of range table entries. For example, `Eref` is displayed for all types of range table entries, but `CTE Name` is displayed only for range table entries of type `cte`.

For more information about range table entries, see the definition of `RangeTblEntry` in `nodes/plannodes.h`.

### F.29.3. Author #

Robert Haas `<rhaas@postgresql.org>`

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