F.2. amcheck
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## F.2. amcheck

The `amcheck` module provides functions that allow you to verify the logical consistency of the structure of relations. If the structure appears to be valid, no error is raised.

The functions verify various invariants in the structure of the representation of particular relations. The correctness of the access method functions behind index scans and other important operations relies on these invariants always holding. For example, certain functions verify, among other things, that all B-Tree pages have items in “logical” order (e.g., for B-Tree indexes on `text`, index tuples should be in collated lexical order). If that particular invariant somehow fails to hold, we can expect binary searches on the affected page to incorrectly guide index scans, resulting in wrong answers to SQL queries.

Verification is performed using the same procedures as those used by index scans themselves, which may be user-defined operator class code. For example, B-Tree index verification relies on comparisons made with one or more B-Tree support function 1 routines. See Section 38.15.3 for details of operator class support functions.

`amcheck` functions may only be used by superusers.

### F.2.1. Functions

`bt_index_check(index regclass, heapallindexed boolean) returns void`

  `bt_index_check` tests that its target, a B-Tree index, respects a variety of invariants. Example usage:

      test=# SELECT bt_index_check(index => c.oid, heapallindexed => i.indisunique),
                     c.relname,
                     c.relpages
      FROM pg_index i
      JOIN pg_opclass op ON i.indclass[0] = op.oid
      JOIN pg_am am ON op.opcmethod = am.oid
      JOIN pg_class c ON i.indexrelid = c.oid
      JOIN pg_namespace n ON c.relnamespace = n.oid
      WHERE am.amname = 'btree' AND n.nspname = 'pg_catalog'
      -- Don't check temp tables, which may be from another session:
      AND c.relpersistence != 't'
      -- Function may throw an error when this is omitted:
      AND c.relkind = 'i' AND i.indisready AND i.indisvalid
      ORDER BY c.relpages DESC LIMIT 10;
       bt_index_check |             relname             | relpages
      ----------------+---------------------------------+----------
                      | pg_depend_reference_index       |       43
                      | pg_depend_depender_index        |       40
                      | pg_proc_proname_args_nsp_index  |       31
                      | pg_description_o_c_o_index      |       21
                      | pg_attribute_relid_attnam_index |       14
                      | pg_proc_oid_index               |       10
                      | pg_attribute_relid_attnum_index |        9
                      | pg_amproc_fam_proc_index        |        5
                      | pg_amop_opr_fam_index           |        5
                      | pg_amop_fam_strat_index         |        5
      (10 rows)

  This example shows a session that performs verification of the 10 largest catalog indexes in the database “test”. Verification of the presence of heap tuples as index tuples is requested for the subset that are unique indexes. Since no error is raised, all indexes tested appear to be logically consistent. Naturally, this query could easily be changed to call `bt_index_check` for every index in the database where verification is supported.

  `bt_index_check` acquires an `AccessShareLock` on the target index and the heap relation it belongs to. This lock mode is the same lock mode acquired on relations by simple `SELECT` statements. `bt_index_check` does not verify invariants that span child/parent relationships, but will verify the presence of all heap tuples as index tuples within the index when `heapallindexed` is `true`. When a routine, lightweight test for corruption is required in a live production environment, using `bt_index_check` often provides the best trade-off between thoroughness of verification and limiting the impact on application performance and availability.

`bt_index_parent_check(index regclass, heapallindexed boolean) returns void`

  `bt_index_parent_check` tests that its target, a B-Tree index, respects a variety of invariants. Optionally, when the `heapallindexed` argument is `true`, the function verifies the presence of all heap tuples that should be found within the index, and that there are no missing downlinks in the index structure. The checks that can be performed by `bt_index_parent_check` are a superset of the checks that can be performed by `bt_index_check`. `bt_index_parent_check` can be thought of as a more thorough variant of `bt_index_check`: unlike `bt_index_check`, `bt_index_parent_check` also checks invariants that span parent/child relationships. `bt_index_parent_check` follows the general convention of raising an error if it finds a logical inconsistency or other problem.

  A `ShareLock` is required on the target index by `bt_index_parent_check` (a `ShareLock` is also acquired on the heap relation). These locks prevent concurrent data modification from `INSERT`, `UPDATE`, and `DELETE` commands. The locks also prevent the underlying relation from being concurrently processed by `VACUUM`, as well as all other utility commands. Note that the function holds locks only while running, not for the entire transaction.

  `bt_index_parent_check`'s additional verification is more likely to detect various pathological cases. These cases may involve an incorrectly implemented B-Tree operator class used by the index that is checked, or, hypothetically, undiscovered bugs in the underlying B-Tree index access method code. Note that `bt_index_parent_check` cannot be used when Hot Standby mode is enabled (i.e., on read-only physical replicas), unlike `bt_index_check`.

### F.2.2. Optional `heapallindexed` verification

When the `heapallindexed` argument to verification functions is `true`, an additional phase of verification is performed against the table associated with the target index relation. This consists of a “dummy” `CREATE INDEX` operation, which checks for the presence of all hypothetical new index tuples against a temporary, in-memory summarizing structure (this is built when needed during the basic first phase of verification). The summarizing structure “fingerprints” every tuple found within the target index. The high level principle behind `heapallindexed` verification is that a new index that is equivalent to the existing, target index must only have entries that can be found in the existing structure.

The additional `heapallindexed` phase adds significant overhead: verification will typically take several times longer. However, there is no change to the relation-level locks acquired when `heapallindexed` verification is performed.

The summarizing structure is bound in size by `maintenance_work_mem`. In order to ensure that there is no more than a 2% probability of failure to detect an inconsistency for each heap tuple that should be represented in the index, approximately 2 bytes of memory are needed per tuple. As less memory is made available per tuple, the probability of missing an inconsistency slowly increases. This approach limits the overhead of verification significantly, while only slightly reducing the probability of detecting a problem, especially for installations where verification is treated as a routine maintenance task. Any single absent or malformed tuple has a new opportunity to be detected with each new verification attempt.

### F.2.3. Using `amcheck` effectively

`amcheck` can be effective at detecting various types of failure modes that data page checksums will always fail to catch. These include:

- Structural inconsistencies caused by incorrect operator class implementations.

  This includes issues caused by the comparison rules of operating system collations changing. Comparisons of datums of a collatable type like `text` must be immutable (just as all comparisons used for B-Tree index scans must be immutable), which implies that operating system collation rules must never change. Though rare, updates to operating system collation rules can cause these issues. More commonly, an inconsistency in the collation order between a master server and a standby server is implicated, possibly because the major operating system version in use is inconsistent. Such inconsistencies will generally only arise on standby servers, and so can generally only be detected on standby servers.

  If a problem like this arises, it may not affect each individual index that is ordered using an affected collation, simply because indexed values might happen to have the same absolute ordering regardless of the behavioral inconsistency. See Section 23.1 and Section 23.2 for further details about how PostgreSQL uses operating system locales and collations.

- Structural inconsistencies between indexes and the heap relations that are indexed (when `heapallindexed` verification is performed).

  There is no cross-checking of indexes against their heap relation during normal operation. Symptoms of heap corruption can be subtle.

- Corruption caused by hypothetical undiscovered bugs in the underlying PostgreSQL access method code, sort code, or transaction management code.

  Automatic verification of the structural integrity of indexes plays a role in the general testing of new or proposed PostgreSQL features that could plausibly allow a logical inconsistency to be introduced. Verification of table structure and associated visibility and transaction status information plays a similar role. One obvious testing strategy is to call `amcheck` functions continuously when running the standard regression tests. See Section 33.1 for details on running the tests.

- File system or storage subsystem faults where checksums happen to simply not be enabled.

  Note that `amcheck` examines a page as represented in some shared memory buffer at the time of verification if there is only a shared buffer hit when accessing the block. Consequently, `amcheck` does not necessarily examine data read from the file system at the time of verification. Note that when checksums are enabled, `amcheck` may raise an error due to a checksum failure when a corrupt block is read into a buffer.

- Corruption caused by faulty RAM, or the broader memory subsystem.

  PostgreSQL does not protect against correctable memory errors and it is assumed you will operate using RAM that uses industry standard Error Correcting Codes (ECC) or better protection. However, ECC memory is typically only immune to single-bit errors, and should not be assumed to provide absolute protection against failures that result in memory corruption.

  When `heapallindexed` verification is performed, there is generally a greatly increased chance of detecting single-bit errors, since strict binary equality is tested, and the indexed attributes within the heap are tested.

In general, `amcheck` can only prove the presence of corruption; it cannot prove its absence.

### F.2.4. Repairing corruption

No error concerning corruption raised by `amcheck` should ever be a false positive. `amcheck` raises errors in the event of conditions that, by definition, should never happen, and so careful analysis of `amcheck` errors is often required.

There is no general method of repairing problems that `amcheck` detects. An explanation for the root cause of an invariant violation should be sought. pageinspect may play a useful role in diagnosing corruption that `amcheck` detects. A `REINDEX` may not be effective in repairing corruption.

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