amcheck module provides
functions that allow you to verify the logical consistency of the
structure of indexes. If the structure appears to be valid, no
error is raised.
The functions verify various invariants in the structure of the
representation of particular indexes. 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 37.14.3 for details of operator class support functions.
amcheck functions may only be used
bt_index_check(index regclass) returns void
bt_index_check tests that its
target, a B-Tree index, respects a variety of invariants. Example
test=# SELECT bt_index_check(c.oid), c.relname, c.relpages FROM pg_index i JOIN pg_opclass op ON i.indclass = 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 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”. 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
bt_index_check does not verify invariants that
span child/parent relationships, nor does it verify that the target
index is consistent with its heap relation. When a routine,
lightweight test for corruption is required in a live production
often provides the best trade-off between thoroughness of
verification and limiting the impact on application performance and
bt_index_parent_check(index regclass) returns void
bt_index_parent_check tests that
its target, a B-Tree index, respects a variety of invariants. The
checks performed by
bt_index_parent_check are a superset of the
checks performed by
bt_index_parent_check can be thought
of as a more thorough variant of
bt_index_parent_check also checks invariants that
span parent/child relationships. However, it does not verify that
the target index is consistent with its heap relation.
bt_index_parent_check follows the general
convention of raising an error if it finds a logical inconsistency
or other problem.
ShareLock is required on the
target index by
ShareLock is also acquired on the
heap relation). These locks prevent concurrent data modification
commands. The locks also prevent the underlying relation from being
concurrently processed by
well as all other utility commands. Note that the function holds
locks only while running, not for the entire transaction.
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
cannot be used when Hot Standby mode is enabled (i.e., on read-only
physical replicas), unlike
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
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
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.
Corruption caused by hypothetical undiscovered bugs in the underlying PostgreSQL access method code or sort 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. One obvious testing
strategy is to call
continuously when running the standard regression tests. See
Section 32.1 for details on
running the tests.
File system or storage subsystem faults where checksums happen to simply not be enabled.
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,
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.
amcheck can only prove
the presence of corruption; it cannot prove its absence.
No error concerning corruption raised by
amcheck should ever be a false positive. In
amcheck is more likely to
find software bugs than problems with hardware.
amcheck raises errors in the event of conditions
that, by definition, should never happen, and so careful analysis
amcheck errors is often
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
REINDEX may not be
effective in repairing corruption.
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