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| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.2. amcheck — tools to verify table and index consistency</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="adminpack.html" title="F.1. adminpack — pgAdmin support toolpack" /><link rel="next" href="auth-delay.html" title="F.3. auth_delay — pause on authentication failure" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.2. amcheck — tools to verify table and index consistency</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="adminpack.html" title="F.1. adminpack — pgAdmin support toolpack">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules and Extensions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="auth-delay.html" title="F.3. auth_delay — pause on authentication failure">Next</a></td></tr></table><hr /></div><div class="sect1" id="AMCHECK"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.2. amcheck — tools to verify table and index consistency <a href="#AMCHECK" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="amcheck.html#AMCHECK-FUNCTIONS">F.2.1. Functions</a></span></dt><dt><span class="sect2"><a href="amcheck.html#AMCHECK-OPTIONAL-HEAPALLINDEXED-VERIFICATION">F.2.2. Optional <em class="parameter"><code>heapallindexed</code></em> Verification</a></span></dt><dt><span class="sect2"><a href="amcheck.html#AMCHECK-USING-AMCHECK-EFFECTIVELY">F.2.3. Using <code class="filename">amcheck</code> Effectively</a></span></dt><dt><span class="sect2"><a href="amcheck.html#AMCHECK-REPAIRING-CORRUPTION">F.2.4. Repairing Corruption</a></span></dt></dl></div><a id="id-1.11.7.12.2" class="indexterm"></a><p> |
| The <code class="filename">amcheck</code> module provides functions that allow you to |
| verify the logical consistency of the structure of relations. |
| </p><p> |
| The B-Tree checking functions verify various <span class="emphasis"><em>invariants</em></span> 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 <span class="quote">“<span class="quote">logical</span>”</span> order (e.g., |
| for B-Tree indexes on <code class="type">text</code>, 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. If the structure appears to be valid, no error is raised. |
| </p><p> |
| 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 <a class="xref" href="xindex.html#XINDEX-SUPPORT" title="38.16.3. Index Method Support Routines">Section 38.16.3</a> for details of operator class support |
| functions. |
| </p><p> |
| Unlike the B-Tree checking functions which report corruption by raising |
| errors, the heap checking function <code class="function">verify_heapam</code> checks |
| a table and attempts to return a set of rows, one row per corruption |
| detected. Despite this, if facilities that |
| <code class="function">verify_heapam</code> relies upon are themselves corrupted, the |
| function may be unable to continue and may instead raise an error. |
| </p><p> |
| Permission to execute <code class="filename">amcheck</code> functions may be granted |
| to non-superusers, but before granting such permissions careful consideration |
| should be given to data security and privacy concerns. Although the |
| corruption reports generated by these functions do not focus on the contents |
| of the corrupted data so much as on the structure of that data and the nature |
| of the corruptions found, an attacker who gains permission to execute these |
| functions, particularly if the attacker can also induce corruption, might be |
| able to infer something of the data itself from such messages. |
| </p><div class="sect2" id="AMCHECK-FUNCTIONS"><div class="titlepage"><div><div><h3 class="title">F.2.1. Functions <a href="#AMCHECK-FUNCTIONS" class="id_link">#</a></h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt><span class="term"> |
| <code class="function">bt_index_check(index regclass, heapallindexed boolean) returns void</code> |
| <a id="id-1.11.7.12.8.2.1.1.2" class="indexterm"></a> |
| </span></dt><dd><p> |
| <code class="function">bt_index_check</code> tests that its target, a |
| B-Tree index, respects a variety of invariants. Example usage: |
| </p><pre class="screen"> |
| 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) |
| </pre><p> |
| This example shows a session that performs verification of the |
| 10 largest catalog indexes in the database <span class="quote">“<span class="quote">test</span>”</span>. |
| 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 <code class="function">bt_index_check</code> for every index in the |
| database where verification is supported. |
| </p><p> |
| <code class="function">bt_index_check</code> acquires an <code class="literal">AccessShareLock</code> |
| on the target index and the heap relation it belongs to. This lock mode |
| is the same lock mode acquired on relations by simple |
| <code class="literal">SELECT</code> statements. |
| <code class="function">bt_index_check</code> 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 <em class="parameter"><code>heapallindexed</code></em> is |
| <code class="literal">true</code>. When a routine, lightweight test for |
| corruption is required in a live production environment, using |
| <code class="function">bt_index_check</code> often provides the best |
| trade-off between thoroughness of verification and limiting the |
| impact on application performance and availability. |
| </p></dd><dt><span class="term"> |
| <code class="function">bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</code> |
| <a id="id-1.11.7.12.8.2.2.1.2" class="indexterm"></a> |
| </span></dt><dd><p> |
| <code class="function">bt_index_parent_check</code> tests that its |
| target, a B-Tree index, respects a variety of invariants. |
| Optionally, when the <em class="parameter"><code>heapallindexed</code></em> |
| argument is <code class="literal">true</code>, the function verifies the |
| presence of all heap tuples that should be found within the |
| index. When the optional <em class="parameter"><code>rootdescend</code></em> |
| argument is <code class="literal">true</code>, 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 |
| <code class="function">bt_index_parent_check</code> are a superset of the |
| checks that can be performed by <code class="function">bt_index_check</code>. |
| <code class="function">bt_index_parent_check</code> can be thought of as |
| a more thorough variant of <code class="function">bt_index_check</code>: |
| unlike <code class="function">bt_index_check</code>, |
| <code class="function">bt_index_parent_check</code> also checks |
| invariants that span parent/child relationships, including checking |
| that there are no missing downlinks in the index structure. |
| <code class="function">bt_index_parent_check</code> follows the general |
| convention of raising an error if it finds a logical |
| inconsistency or other problem. |
| </p><p> |
| A <code class="literal">ShareLock</code> is required on the target index by |
| <code class="function">bt_index_parent_check</code> (a |
| <code class="literal">ShareLock</code> is also acquired on the heap relation). |
| These locks prevent concurrent data modification from |
| <code class="command">INSERT</code>, <code class="command">UPDATE</code>, and <code class="command">DELETE</code> |
| commands. The locks also prevent the underlying relation from |
| being concurrently processed by <code class="command">VACUUM</code>, as well as |
| all other utility commands. Note that the function holds locks |
| only while running, not for the entire transaction. |
| </p><p> |
| <code class="function">bt_index_parent_check</code>'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 |
| <code class="function">bt_index_parent_check</code> cannot be used when |
| hot standby mode is enabled (i.e., on read-only physical |
| replicas), unlike <code class="function">bt_index_check</code>. |
| </p></dd></dl></div><div class="tip"><h3 class="title">Tip</h3><p> |
| <code class="function">bt_index_check</code> and |
| <code class="function">bt_index_parent_check</code> both output log |
| messages about the verification process at |
| <code class="literal">DEBUG1</code> and <code class="literal">DEBUG2</code> severity |
| levels. These messages provide detailed information about the |
| verification process that may be of interest to |
| <span class="productname">PostgreSQL</span> developers. Advanced users |
| may also find this information helpful, since it provides |
| additional context should verification actually detect an |
| inconsistency. Running: |
| </p><pre class="programlisting"> |
| SET client_min_messages = DEBUG1; |
| </pre><p> |
| in an interactive <span class="application">psql</span> session before |
| running a verification query will display messages about the |
| progress of verification with a manageable level of detail. |
| </p></div><div class="variablelist"><dl class="variablelist"><dt><span class="term"> |
| <code class="function"> |
| verify_heapam(relation regclass, |
| on_error_stop boolean, |
| check_toast boolean, |
| skip text, |
| startblock bigint, |
| endblock bigint, |
| blkno OUT bigint, |
| offnum OUT integer, |
| attnum OUT integer, |
| msg OUT text) |
| returns setof record |
| </code> |
| </span></dt><dd><p> |
| Checks a table, sequence, or materialized view for structural corruption, |
| where pages in the relation contain data that is invalidly formatted, and |
| for logical corruption, where pages are structurally valid but |
| inconsistent with the rest of the database cluster. |
| </p><p> |
| The following optional arguments are recognized: |
| </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">on_error_stop</code></span></dt><dd><p> |
| If true, corruption checking stops at the end of the first block in |
| which any corruptions are found. |
| </p><p> |
| Defaults to false. |
| </p></dd><dt><span class="term"><code class="literal">check_toast</code></span></dt><dd><p> |
| If true, toasted values are checked against the target relation's |
| TOAST table. |
| </p><p> |
| This option is known to be slow. Also, if the toast table or its |
| index is corrupt, checking it against toast values could conceivably |
| crash the server, although in many cases this would just produce an |
| error. |
| </p><p> |
| Defaults to false. |
| </p></dd><dt><span class="term"><code class="literal">skip</code></span></dt><dd><p> |
| If not <code class="literal">none</code>, corruption checking skips blocks that |
| are marked as all-visible or all-frozen, as specified. |
| Valid options are <code class="literal">all-visible</code>, |
| <code class="literal">all-frozen</code> and <code class="literal">none</code>. |
| </p><p> |
| Defaults to <code class="literal">none</code>. |
| </p></dd><dt><span class="term"><code class="literal">startblock</code></span></dt><dd><p> |
| If specified, corruption checking begins at the specified block, |
| skipping all previous blocks. It is an error to specify a |
| <em class="parameter"><code>startblock</code></em> outside the range of blocks in the |
| target table. |
| </p><p> |
| By default, checking begins at the first block. |
| </p></dd><dt><span class="term"><code class="literal">endblock</code></span></dt><dd><p> |
| If specified, corruption checking ends at the specified block, |
| skipping all remaining blocks. It is an error to specify an |
| <em class="parameter"><code>endblock</code></em> outside the range of blocks in the target |
| table. |
| </p><p> |
| By default, all blocks are checked. |
| </p></dd></dl></div><p> |
| For each corruption detected, <code class="function">verify_heapam</code> returns |
| a row with the following columns: |
| </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">blkno</code></span></dt><dd><p> |
| The number of the block containing the corrupt page. |
| </p></dd><dt><span class="term"><code class="literal">offnum</code></span></dt><dd><p> |
| The OffsetNumber of the corrupt tuple. |
| </p></dd><dt><span class="term"><code class="literal">attnum</code></span></dt><dd><p> |
| The attribute number of the corrupt column in the tuple, if the |
| corruption is specific to a column and not the tuple as a whole. |
| </p></dd><dt><span class="term"><code class="literal">msg</code></span></dt><dd><p> |
| A message describing the problem detected. |
| </p></dd></dl></div></dd></dl></div></div><div class="sect2" id="AMCHECK-OPTIONAL-HEAPALLINDEXED-VERIFICATION"><div class="titlepage"><div><div><h3 class="title">F.2.2. Optional <em class="parameter"><code>heapallindexed</code></em> Verification <a href="#AMCHECK-OPTIONAL-HEAPALLINDEXED-VERIFICATION" class="id_link">#</a></h3></div></div></div><p> |
| When the <em class="parameter"><code>heapallindexed</code></em> argument to B-Tree |
| verification functions is <code class="literal">true</code>, an additional |
| phase of verification is performed against the table associated with |
| the target index relation. This consists of a <span class="quote">“<span class="quote">dummy</span>”</span> |
| <code class="command">CREATE INDEX</code> 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 |
| <span class="quote">“<span class="quote">fingerprints</span>”</span> every tuple found within the target |
| index. The high level principle behind |
| <em class="parameter"><code>heapallindexed</code></em> 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. |
| </p><p> |
| The additional <em class="parameter"><code>heapallindexed</code></em> phase adds |
| significant overhead: verification will typically take several times |
| longer. However, there is no change to the relation-level locks |
| acquired when <em class="parameter"><code>heapallindexed</code></em> verification is |
| performed. |
| </p><p> |
| The summarizing structure is bound in size by |
| <code class="varname">maintenance_work_mem</code>. 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. |
| </p></div><div class="sect2" id="AMCHECK-USING-AMCHECK-EFFECTIVELY"><div class="titlepage"><div><div><h3 class="title">F.2.3. Using <code class="filename">amcheck</code> Effectively <a href="#AMCHECK-USING-AMCHECK-EFFECTIVELY" class="id_link">#</a></h3></div></div></div><p> |
| <code class="filename">amcheck</code> can be effective at detecting various types of |
| failure modes that <a class="link" href="app-initdb.html#APP-INITDB-DATA-CHECKSUMS"><span class="application">data |
| checksums</span></a> will fail to catch. These include: |
|
|
| </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> |
| Structural inconsistencies caused by incorrect operator class |
| implementations. |
| </p><p> |
| This includes issues caused by the comparison rules of operating |
| system collations changing. Comparisons of datums of a collatable |
| type like <code class="type">text</code> 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 primary server and a standby server is |
| implicated, possibly because the <span class="emphasis"><em>major</em></span> 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. |
| </p><p> |
| If a problem like this arises, it may not affect each individual |
| index that is ordered using an affected collation, simply because |
| <span class="emphasis"><em>indexed</em></span> values might happen to have the same |
| absolute ordering regardless of the behavioral inconsistency. See |
| <a class="xref" href="locale.html" title="24.1. Locale Support">Section 24.1</a> and <a class="xref" href="collation.html" title="24.2. Collation Support">Section 24.2</a> for |
| further details about how <span class="productname">PostgreSQL</span> uses |
| operating system locales and collations. |
| </p></li><li class="listitem"><p> |
| Structural inconsistencies between indexes and the heap relations |
| that are indexed (when <em class="parameter"><code>heapallindexed</code></em> |
| verification is performed). |
| </p><p> |
| There is no cross-checking of indexes against their heap relation |
| during normal operation. Symptoms of heap corruption can be subtle. |
| </p></li><li class="listitem"><p> |
| Corruption caused by hypothetical undiscovered bugs in the |
| underlying <span class="productname">PostgreSQL</span> access method |
| code, sort code, or transaction management code. |
| </p><p> |
| Automatic verification of the structural integrity of indexes |
| plays a role in the general testing of new or proposed |
| <span class="productname">PostgreSQL</span> 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 <code class="filename">amcheck</code> functions continuously |
| when running the standard regression tests. See <a class="xref" href="regress-run.html" title="33.1. Running the Tests">Section 33.1</a> for details on running the tests. |
| </p></li><li class="listitem"><p> |
| File system or storage subsystem faults where checksums happen to |
| simply not be enabled. |
| </p><p> |
| Note that <code class="filename">amcheck</code> 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, |
| <code class="filename">amcheck</code> does not necessarily examine data read from the |
| file system at the time of verification. Note that when checksums are |
| enabled, <code class="filename">amcheck</code> may raise an error due to a checksum |
| failure when a corrupt block is read into a buffer. |
| </p></li><li class="listitem"><p> |
| Corruption caused by faulty RAM, or the broader memory subsystem. |
| </p><p> |
| <span class="productname">PostgreSQL</span> 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 |
| <span class="emphasis"><em>absolute</em></span> protection against failures that |
| result in memory corruption. |
| </p><p> |
| When <em class="parameter"><code>heapallindexed</code></em> 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. |
| </p></li></ul></div><p> |
| </p><p> |
| Structural corruption can happen due to faulty storage hardware, or |
| relation files being overwritten or modified by unrelated software. |
| This kind of corruption can also be detected with |
| <a class="link" href="checksums.html" title="30.2. Data Checksums"><span class="application">data page |
| checksums</span></a>. |
| </p><p> |
| Relation pages which are correctly formatted, internally consistent, and |
| correct relative to their own internal checksums may still contain |
| logical corruption. As such, this kind of corruption cannot be detected |
| with <span class="application">checksums</span>. Examples include toasted |
| values in the main table which lack a corresponding entry in the toast |
| table, and tuples in the main table with a Transaction ID that is older |
| than the oldest valid Transaction ID in the database or cluster. |
| </p><p> |
| Multiple causes of logical corruption have been observed in production |
| systems, including bugs in the <span class="productname">PostgreSQL</span> |
| server software, faulty and ill-conceived backup and restore tools, and |
| user error. |
| </p><p> |
| Corrupt relations are most concerning in live production environments, |
| precisely the same environments where high risk activities are least |
| welcome. For this reason, <code class="function">verify_heapam</code> has been |
| designed to diagnose corruption without undue risk. It cannot guard |
| against all causes of backend crashes, as even executing the calling |
| query could be unsafe on a badly corrupted system. Access to <a class="link" href="catalogs-overview.html" title="53.1. Overview">catalog tables</a> is performed and could |
| be problematic if the catalogs themselves are corrupted. |
| </p><p> |
| In general, <code class="filename">amcheck</code> can only prove the presence of |
| corruption; it cannot prove its absence. |
| </p></div><div class="sect2" id="AMCHECK-REPAIRING-CORRUPTION"><div class="titlepage"><div><div><h3 class="title">F.2.4. Repairing Corruption <a href="#AMCHECK-REPAIRING-CORRUPTION" class="id_link">#</a></h3></div></div></div><p> |
| No error concerning corruption raised by <code class="filename">amcheck</code> should |
| ever be a false positive. <code class="filename">amcheck</code> raises |
| errors in the event of conditions that, by definition, should never |
| happen, and so careful analysis of <code class="filename">amcheck</code> |
| errors is often required. |
| </p><p> |
| There is no general method of repairing problems that |
| <code class="filename">amcheck</code> detects. An explanation for the root cause of |
| an invariant violation should be sought. <a class="xref" href="pageinspect.html" title="F.25. pageinspect — low-level inspection of database pages">pageinspect</a> may play a useful role in diagnosing |
| corruption that <code class="filename">amcheck</code> detects. A <code class="command">REINDEX</code> |
| may not be effective in repairing corruption. |
| </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="adminpack.html" title="F.1. adminpack — pgAdmin support toolpack">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="auth-delay.html" title="F.3. auth_delay — pause on authentication failure">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.1. adminpack — pgAdmin support toolpack </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.3. auth_delay — pause on authentication failure</td></tr></table></div></body></html> |
