F.2. amcheck

F.2. amcheck
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F.2.1. Functions
F.2.2. Optional heapallindexed Verification
F.2.3. Using amcheck Effectively
F.2.4. Repairing Corruption

+ 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 37.16.3 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, rootdescend 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. When the optional rootdescend + argument is true, verification re-finds + tuples on the leaf level by performing a new search from the + root page for each tuple. 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, including checking + that there are no missing downlinks in the index structure. + 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. +

Tip

+ bt_index_check and + bt_index_parent_check both output log + messages about the verification process at + DEBUG1 and DEBUG2 severity + levels. These messages provide detailed information about the + verification process that may be of interest to + PostgreSQL developers. Advanced users + may also find this information helpful, since it provides + additional context should verification actually detect an + inconsistency. Running: +

+SET client_min_messages = DEBUG1;
+

+ in an interactive psql session before + running a verification query will display messages about the + progress of verification with a manageable level of detail. +

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 32.1 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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F.2. amcheck

F.2.1. Functions

Tip

F.2.2. Optional heapallindexed Verification

F.2.3. Using amcheck Effectively

F.2.4. Repairing Corruption

F.2.2. Optional `heapallindexed` Verification

F.2.3. Using `amcheck` Effectively