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diff --git a/doc/src/sgml/btree.sgml b/doc/src/sgml/btree.sgml new file mode 100644 index 0000000..6f608a1 --- /dev/null +++ b/doc/src/sgml/btree.sgml @@ -0,0 +1,914 @@ +<!-- doc/src/sgml/btree.sgml --> + +<chapter id="btree"> +<title>B-Tree Indexes</title> + + <indexterm> + <primary>index</primary> + <secondary>B-Tree</secondary> + </indexterm> + +<sect1 id="btree-intro"> + <title>Introduction</title> + + <para> + <productname>PostgreSQL</productname> includes an implementation of the + standard <acronym>btree</acronym> (multi-way balanced tree) index data + structure. Any data type that can be sorted into a well-defined linear + order can be indexed by a btree index. The only limitation is that an + index entry cannot exceed approximately one-third of a page (after TOAST + compression, if applicable). + </para> + + <para> + Because each btree operator class imposes a sort order on its data type, + btree operator classes (or, really, operator families) have come to be + used as <productname>PostgreSQL</productname>'s general representation + and understanding of sorting semantics. Therefore, they've acquired + some features that go beyond what would be needed just to support btree + indexes, and parts of the system that are quite distant from the + btree AM make use of them. + </para> + +</sect1> + +<sect1 id="btree-behavior"> + <title>Behavior of B-Tree Operator Classes</title> + + <para> + As shown in <xref linkend="xindex-btree-strat-table"/>, a btree operator + class must provide five comparison operators, + <literal><</literal>, + <literal><=</literal>, + <literal>=</literal>, + <literal>>=</literal> and + <literal>></literal>. + One might expect that <literal><></literal> should also be part of + the operator class, but it is not, because it would almost never be + useful to use a <literal><></literal> WHERE clause in an index + search. (For some purposes, the planner treats <literal><></literal> + as associated with a btree operator class; but it finds that operator via + the <literal>=</literal> operator's negator link, rather than + from <structname>pg_amop</structname>.) + </para> + + <para> + When several data types share near-identical sorting semantics, their + operator classes can be grouped into an operator family. Doing so is + advantageous because it allows the planner to make deductions about + cross-type comparisons. Each operator class within the family should + contain the single-type operators (and associated support functions) + for its input data type, while cross-type comparison operators and + support functions are <quote>loose</quote> in the family. It is + recommendable that a complete set of cross-type operators be included + in the family, thus ensuring that the planner can represent any + comparison conditions that it deduces from transitivity. + </para> + + <para> + There are some basic assumptions that a btree operator family must + satisfy: + </para> + + <itemizedlist> + <listitem> + <para> + An <literal>=</literal> operator must be an equivalence relation; that + is, for all non-null values <replaceable>A</replaceable>, + <replaceable>B</replaceable>, <replaceable>C</replaceable> of the + data type: + + <itemizedlist> + <listitem> + <para> + <replaceable>A</replaceable> <literal>=</literal> + <replaceable>A</replaceable> is true + (<firstterm>reflexive law</firstterm>) + </para> + </listitem> + <listitem> + <para> + if <replaceable>A</replaceable> <literal>=</literal> + <replaceable>B</replaceable>, + then <replaceable>B</replaceable> <literal>=</literal> + <replaceable>A</replaceable> + (<firstterm>symmetric law</firstterm>) + </para> + </listitem> + <listitem> + <para> + if <replaceable>A</replaceable> <literal>=</literal> + <replaceable>B</replaceable> and <replaceable>B</replaceable> + <literal>=</literal> <replaceable>C</replaceable>, + then <replaceable>A</replaceable> <literal>=</literal> + <replaceable>C</replaceable> + (<firstterm>transitive law</firstterm>) + </para> + </listitem> + </itemizedlist> + </para> + </listitem> + + <listitem> + <para> + A <literal><</literal> operator must be a strong ordering relation; + that is, for all non-null values <replaceable>A</replaceable>, + <replaceable>B</replaceable>, <replaceable>C</replaceable>: + + <itemizedlist> + <listitem> + <para> + <replaceable>A</replaceable> <literal><</literal> + <replaceable>A</replaceable> is false + (<firstterm>irreflexive law</firstterm>) + </para> + </listitem> + <listitem> + <para> + if <replaceable>A</replaceable> <literal><</literal> + <replaceable>B</replaceable> + and <replaceable>B</replaceable> <literal><</literal> + <replaceable>C</replaceable>, + then <replaceable>A</replaceable> <literal><</literal> + <replaceable>C</replaceable> + (<firstterm>transitive law</firstterm>) + </para> + </listitem> + </itemizedlist> + </para> + </listitem> + + <listitem> + <para> + Furthermore, the ordering is total; that is, for all non-null + values <replaceable>A</replaceable>, <replaceable>B</replaceable>: + + <itemizedlist> + <listitem> + <para> + exactly one of <replaceable>A</replaceable> <literal><</literal> + <replaceable>B</replaceable>, <replaceable>A</replaceable> + <literal>=</literal> <replaceable>B</replaceable>, and + <replaceable>B</replaceable> <literal><</literal> + <replaceable>A</replaceable> is true + (<firstterm>trichotomy law</firstterm>) + </para> + </listitem> + </itemizedlist> + + (The trichotomy law justifies the definition of the comparison support + function, of course.) + </para> + </listitem> + </itemizedlist> + + <para> + The other three operators are defined in terms of <literal>=</literal> + and <literal><</literal> in the obvious way, and must act consistently + with them. + </para> + + <para> + For an operator family supporting multiple data types, the above laws must + hold when <replaceable>A</replaceable>, <replaceable>B</replaceable>, + <replaceable>C</replaceable> are taken from any data types in the family. + The transitive laws are the trickiest to ensure, as in cross-type + situations they represent statements that the behaviors of two or three + different operators are consistent. + As an example, it would not work to put <type>float8</type> + and <type>numeric</type> into the same operator family, at least not with + the current semantics that <type>numeric</type> values are converted + to <type>float8</type> for comparison to a <type>float8</type>. Because + of the limited accuracy of <type>float8</type>, this means there are + distinct <type>numeric</type> values that will compare equal to the + same <type>float8</type> value, and thus the transitive law would fail. + </para> + + <para> + Another requirement for a multiple-data-type family is that any implicit + or binary-coercion casts that are defined between data types included in + the operator family must not change the associated sort ordering. + </para> + + <para> + It should be fairly clear why a btree index requires these laws to hold + within a single data type: without them there is no ordering to arrange + the keys with. Also, index searches using a comparison key of a + different data type require comparisons to behave sanely across two + data types. The extensions to three or more data types within a family + are not strictly required by the btree index mechanism itself, but the + planner relies on them for optimization purposes. + </para> + +</sect1> + +<sect1 id="btree-support-funcs"> + <title>B-Tree Support Functions</title> + + <para> + As shown in <xref linkend="xindex-btree-support-table"/>, btree defines + one required and four optional support functions. The five + user-defined methods are: + </para> + <variablelist> + <varlistentry> + <term><function>order</function></term> + <listitem> + <para> + For each combination of data types that a btree operator family + provides comparison operators for, it must provide a comparison + support function, registered in + <structname>pg_amproc</structname> with support function number 1 + and + <structfield>amproclefttype</structfield>/<structfield>amprocrighttype</structfield> + equal to the left and right data types for the comparison (i.e., + the same data types that the matching operators are registered + with in <structname>pg_amop</structname>). The comparison + function must take two non-null values + <replaceable>A</replaceable> and <replaceable>B</replaceable> and + return an <type>int32</type> value that is + <literal><</literal> <literal>0</literal>, + <literal>0</literal>, or <literal>></literal> + <literal>0</literal> when <replaceable>A</replaceable> + <literal><</literal> <replaceable>B</replaceable>, + <replaceable>A</replaceable> <literal>=</literal> + <replaceable>B</replaceable>, or <replaceable>A</replaceable> + <literal>></literal> <replaceable>B</replaceable>, + respectively. A null result is disallowed: all values of the + data type must be comparable. See + <filename>src/backend/access/nbtree/nbtcompare.c</filename> for + examples. + </para> + + <para> + If the compared values are of a collatable data type, the + appropriate collation OID will be passed to the comparison + support function, using the standard + <function>PG_GET_COLLATION()</function> mechanism. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><function>sortsupport</function></term> + <listitem> + <para> + Optionally, a btree operator family may provide <firstterm>sort + support</firstterm> function(s), registered under support + function number 2. These functions allow implementing + comparisons for sorting purposes in a more efficient way than + naively calling the comparison support function. The APIs + involved in this are defined in + <filename>src/include/utils/sortsupport.h</filename>. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><function>in_range</function></term> + <listitem> + <indexterm> + <primary>in_range support functions</primary> + </indexterm> + + <indexterm> + <primary>support functions</primary> + <secondary>in_range</secondary> + </indexterm> + <para> + Optionally, a btree operator family may provide + <firstterm>in_range</firstterm> support function(s), registered + under support function number 3. These are not used during btree + index operations; rather, they extend the semantics of the + operator family so that it can support window clauses containing + the <literal>RANGE</literal> <replaceable>offset</replaceable> + <literal>PRECEDING</literal> and <literal>RANGE</literal> + <replaceable>offset</replaceable> <literal>FOLLOWING</literal> + frame bound types (see <xref + linkend="syntax-window-functions"/>). Fundamentally, the extra + information provided is how to add or subtract an + <replaceable>offset</replaceable> value in a way that is + compatible with the family's data ordering. + </para> + + <para> + An <function>in_range</function> function must have the signature +<synopsis> +in_range(<replaceable>val</replaceable> type1, <replaceable>base</replaceable> type1, <replaceable>offset</replaceable> type2, <replaceable>sub</replaceable> bool, <replaceable>less</replaceable> bool) +returns bool +</synopsis> + <replaceable>val</replaceable> and + <replaceable>base</replaceable> must be of the same type, which + is one of the types supported by the operator family (i.e., a + type for which it provides an ordering). However, + <replaceable>offset</replaceable> could be of a different type, + which might be one otherwise unsupported by the family. An + example is that the built-in <literal>time_ops</literal> family + provides an <function>in_range</function> function that has + <replaceable>offset</replaceable> of type <type>interval</type>. + A family can provide <function>in_range</function> functions for + any of its supported types and one or more + <replaceable>offset</replaceable> types. Each + <function>in_range</function> function should be entered in + <structname>pg_amproc</structname> with + <structfield>amproclefttype</structfield> equal to + <type>type1</type> and <structfield>amprocrighttype</structfield> + equal to <type>type2</type>. + </para> + + <para> + The essential semantics of an <function>in_range</function> + function depend on the two Boolean flag parameters. It should + add or subtract <replaceable>base</replaceable> and + <replaceable>offset</replaceable>, then compare + <replaceable>val</replaceable> to the result, as follows: + <itemizedlist> + <listitem> + <para> + if <literal>!</literal><replaceable>sub</replaceable> and + <literal>!</literal><replaceable>less</replaceable>, return + <replaceable>val</replaceable> <literal>>=</literal> + (<replaceable>base</replaceable> <literal>+</literal> + <replaceable>offset</replaceable>) + </para> + </listitem> + <listitem> + <para> + if <literal>!</literal><replaceable>sub</replaceable> and + <replaceable>less</replaceable>, return + <replaceable>val</replaceable> <literal><=</literal> + (<replaceable>base</replaceable> <literal>+</literal> + <replaceable>offset</replaceable>) + </para> + </listitem> + <listitem> + <para> + if <replaceable>sub</replaceable> and + <literal>!</literal><replaceable>less</replaceable>, return + <replaceable>val</replaceable> <literal>>=</literal> + (<replaceable>base</replaceable> <literal>-</literal> + <replaceable>offset</replaceable>) + </para> + </listitem> + <listitem> + <para> + if <replaceable>sub</replaceable> and + <replaceable>less</replaceable>, return + <replaceable>val</replaceable> <literal><=</literal> + (<replaceable>base</replaceable> <literal>-</literal> + <replaceable>offset</replaceable>) + </para> + </listitem> + </itemizedlist> + Before doing so, the function should check the sign of + <replaceable>offset</replaceable>: if it is less than zero, raise + error + <literal>ERRCODE_INVALID_PRECEDING_OR_FOLLOWING_SIZE</literal> + (22013) with error text like <quote>invalid preceding or + following size in window function</quote>. (This is required by + the SQL standard, although nonstandard operator families might + perhaps choose to ignore this restriction, since there seems to + be little semantic necessity for it.) This requirement is + delegated to the <function>in_range</function> function so that + the core code needn't understand what <quote>less than + zero</quote> means for a particular data type. + </para> + + <para> + An additional expectation is that <function>in_range</function> + functions should, if practical, avoid throwing an error if + <replaceable>base</replaceable> <literal>+</literal> + <replaceable>offset</replaceable> or + <replaceable>base</replaceable> <literal>-</literal> + <replaceable>offset</replaceable> would overflow. The correct + comparison result can be determined even if that value would be + out of the data type's range. Note that if the data type + includes concepts such as <quote>infinity</quote> or + <quote>NaN</quote>, extra care may be needed to ensure that + <function>in_range</function>'s results agree with the normal + sort order of the operator family. + </para> + + <para> + The results of the <function>in_range</function> function must be + consistent with the sort ordering imposed by the operator family. + To be precise, given any fixed values of + <replaceable>offset</replaceable> and + <replaceable>sub</replaceable>, then: + <itemizedlist> + <listitem> + <para> + If <function>in_range</function> with + <replaceable>less</replaceable> = true is true for some + <replaceable>val1</replaceable> and + <replaceable>base</replaceable>, it must be true for every + <replaceable>val2</replaceable> <literal><=</literal> + <replaceable>val1</replaceable> with the same + <replaceable>base</replaceable>. + </para> + </listitem> + <listitem> + <para> + If <function>in_range</function> with + <replaceable>less</replaceable> = true is false for some + <replaceable>val1</replaceable> and + <replaceable>base</replaceable>, it must be false for every + <replaceable>val2</replaceable> <literal>>=</literal> + <replaceable>val1</replaceable> with the same + <replaceable>base</replaceable>. + </para> + </listitem> + <listitem> + <para> + If <function>in_range</function> with + <replaceable>less</replaceable> = true is true for some + <replaceable>val</replaceable> and + <replaceable>base1</replaceable>, it must be true for every + <replaceable>base2</replaceable> <literal>>=</literal> + <replaceable>base1</replaceable> with the same + <replaceable>val</replaceable>. + </para> + </listitem> + <listitem> + <para> + If <function>in_range</function> with + <replaceable>less</replaceable> = true is false for some + <replaceable>val</replaceable> and + <replaceable>base1</replaceable>, it must be false for every + <replaceable>base2</replaceable> <literal><=</literal> + <replaceable>base1</replaceable> with the same + <replaceable>val</replaceable>. + </para> + </listitem> + </itemizedlist> + Analogous statements with inverted conditions hold when + <replaceable>less</replaceable> = false. + </para> + + <para> + If the type being ordered (<type>type1</type>) is collatable, the + appropriate collation OID will be passed to the + <function>in_range</function> function, using the standard + PG_GET_COLLATION() mechanism. + </para> + + <para> + <function>in_range</function> functions need not handle NULL + inputs, and typically will be marked strict. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><function>equalimage</function></term> + <listitem> + <para> + Optionally, a btree operator family may provide + <function>equalimage</function> (<quote>equality implies image + equality</quote>) support functions, registered under support + function number 4. These functions allow the core code to + determine when it is safe to apply the btree deduplication + optimization. Currently, <function>equalimage</function> + functions are only called when building or rebuilding an index. + </para> + <para> + An <function>equalimage</function> function must have the + signature +<synopsis> +equalimage(<replaceable>opcintype</replaceable> <type>oid</type>) returns bool +</synopsis> + The return value is static information about an operator class + and collation. Returning <literal>true</literal> indicates that + the <function>order</function> function for the operator class is + guaranteed to only return <literal>0</literal> (<quote>arguments + are equal</quote>) when its <replaceable>A</replaceable> and + <replaceable>B</replaceable> arguments are also interchangeable + without any loss of semantic information. Not registering an + <function>equalimage</function> function or returning + <literal>false</literal> indicates that this condition cannot be + assumed to hold. + </para> + <para> + The <replaceable>opcintype</replaceable> argument is the + <literal><structname>pg_type</structname>.oid</literal> of the + data type that the operator class indexes. This is a convenience + that allows reuse of the same underlying + <function>equalimage</function> function across operator classes. + If <replaceable>opcintype</replaceable> is a collatable data + type, the appropriate collation OID will be passed to the + <function>equalimage</function> function, using the standard + <function>PG_GET_COLLATION()</function> mechanism. + </para> + <para> + As far as the operator class is concerned, returning + <literal>true</literal> indicates that deduplication is safe (or + safe for the collation whose OID was passed to its + <function>equalimage</function> function). However, the core + code will only deem deduplication safe for an index when + <emphasis>every</emphasis> indexed column uses an operator class + that registers an <function>equalimage</function> function, and + each function actually returns <literal>true</literal> when + called. + </para> + <para> + Image equality is <emphasis>almost</emphasis> the same condition + as simple bitwise equality. There is one subtle difference: When + indexing a varlena data type, the on-disk representation of two + image equal datums may not be bitwise equal due to inconsistent + application of <acronym>TOAST</acronym> compression on input. + Formally, when an operator class's + <function>equalimage</function> function returns + <literal>true</literal>, it is safe to assume that the + <literal>datum_image_eq()</literal> C function will always agree + with the operator class's <function>order</function> function + (provided that the same collation OID is passed to both the + <function>equalimage</function> and <function>order</function> + functions). + </para> + <para> + The core code is fundamentally unable to deduce anything about + the <quote>equality implies image equality</quote> status of an + operator class within a multiple-data-type family based on + details from other operator classes in the same family. Also, it + is not sensible for an operator family to register a cross-type + <function>equalimage</function> function, and attempting to do so + will result in an error. This is because <quote>equality implies + image equality</quote> status does not just depend on + sorting/equality semantics, which are more or less defined at the + operator family level. In general, the semantics that one + particular data type implements must be considered separately. + </para> + <para> + The convention followed by the operator classes included with the + core <productname>PostgreSQL</productname> distribution is to + register a stock, generic <function>equalimage</function> + function. Most operator classes register + <function>btequalimage()</function>, which indicates that + deduplication is safe unconditionally. Operator classes for + collatable data types such as <type>text</type> register + <function>btvarstrequalimage()</function>, which indicates that + deduplication is safe with deterministic collations. Best + practice for third-party extensions is to register their own + custom function to retain control. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><function>options</function></term> + <listitem> + <para> + Optionally, a B-tree operator family may provide + <function>options</function> (<quote>operator class specific + options</quote>) support functions, registered under support + function number 5. These functions define a set of user-visible + parameters that control operator class behavior. + </para> + <para> + An <function>options</function> support function must have the + signature +<synopsis> +options(<replaceable>relopts</replaceable> <type>local_relopts *</type>) returns void +</synopsis> + The function is passed a pointer to a <structname>local_relopts</structname> + struct, which needs to be filled with a set of operator class + specific options. The options can be accessed from other support + functions using the <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and + <literal>PG_GET_OPCLASS_OPTIONS()</literal> macros. + </para> + <para> + Currently, no B-Tree operator class has an <function>options</function> + support function. B-tree doesn't allow flexible representation of keys + like GiST, SP-GiST, GIN and BRIN do. So, <function>options</function> + probably doesn't have much application in the current B-tree index + access method. Nevertheless, this support function was added to B-tree + for uniformity, and will probably find uses during further + evolution of B-tree in <productname>PostgreSQL</productname>. + </para> + </listitem> + </varlistentry> + </variablelist> + +</sect1> + +<sect1 id="btree-implementation"> + <title>Implementation</title> + + <para> + This section covers B-Tree index implementation details that may be + of use to advanced users. See + <filename>src/backend/access/nbtree/README</filename> in the source + distribution for a much more detailed, internals-focused description + of the B-Tree implementation. + </para> + <sect2 id="btree-structure"> + <title>B-Tree Structure</title> + <para> + <productname>PostgreSQL</productname> B-Tree indexes are + multi-level tree structures, where each level of the tree can be + used as a doubly-linked list of pages. A single metapage is stored + in a fixed position at the start of the first segment file of the + index. All other pages are either leaf pages or internal pages. + Leaf pages are the pages on the lowest level of the tree. All + other levels consist of internal pages. Each leaf page contains + tuples that point to table rows. Each internal page contains + tuples that point to the next level down in the tree. Typically, + over 99% of all pages are leaf pages. Both internal pages and leaf + pages use the standard page format described in <xref + linkend="storage-page-layout"/>. + </para> + <para> + New leaf pages are added to a B-Tree index when an existing leaf + page cannot fit an incoming tuple. A <firstterm>page + split</firstterm> operation makes room for items that originally + belonged on the overflowing page by moving a portion of the items + to a new page. Page splits must also insert a new + <firstterm>downlink</firstterm> to the new page in the parent page, + which may cause the parent to split in turn. Page splits + <quote>cascade upwards</quote> in a recursive fashion. When the + root page finally cannot fit a new downlink, a <firstterm>root page + split</firstterm> operation takes place. This adds a new level to + the tree structure by creating a new root page that is one level + above the original root page. + </para> + </sect2> + + <sect2 id="btree-deletion"> + <title>Bottom-up Index Deletion</title> + <para> + B-Tree indexes are not directly aware that under MVCC, there might + be multiple extant versions of the same logical table row; to an + index, each tuple is an independent object that needs its own index + entry. <quote>Version churn</quote> tuples may sometimes + accumulate and adversely affect query latency and throughput. This + typically occurs with <command>UPDATE</command>-heavy workloads + where most individual updates cannot apply the + <link linkend="storage-hot"><acronym>HOT</acronym> optimization.</link> + Changing the value of only + one column covered by one index during an <command>UPDATE</command> + <emphasis>always</emphasis> necessitates a new set of index tuples + — one for <emphasis>each and every</emphasis> index on the + table. Note in particular that this includes indexes that were not + <quote>logically modified</quote> by the <command>UPDATE</command>. + All indexes will need a successor physical index tuple that points + to the latest version in the table. Each new tuple within each + index will generally need to coexist with the original + <quote>updated</quote> tuple for a short period of time (typically + until shortly after the <command>UPDATE</command> transaction + commits). + </para> + <para> + B-Tree indexes incrementally delete version churn index tuples by + performing <firstterm>bottom-up index deletion</firstterm> passes. + Each deletion pass is triggered in reaction to an anticipated + <quote>version churn page split</quote>. This only happens with + indexes that are not logically modified by + <command>UPDATE</command> statements, where concentrated build up + of obsolete versions in particular pages would occur otherwise. A + page split will usually be avoided, though it's possible that + certain implementation-level heuristics will fail to identify and + delete even one garbage index tuple (in which case a page split or + deduplication pass resolves the issue of an incoming new tuple not + fitting on a leaf page). The worst-case number of versions that + any index scan must traverse (for any single logical row) is an + important contributor to overall system responsiveness and + throughput. A bottom-up index deletion pass targets suspected + garbage tuples in a single leaf page based on + <emphasis>qualitative</emphasis> distinctions involving logical + rows and versions. This contrasts with the <quote>top-down</quote> + index cleanup performed by autovacuum workers, which is triggered + when certain <emphasis>quantitative</emphasis> table-level + thresholds are exceeded (see <xref linkend="autovacuum"/>). + </para> + <note> + <para> + Not all deletion operations that are performed within B-Tree + indexes are bottom-up deletion operations. There is a distinct + category of index tuple deletion: <firstterm>simple index tuple + deletion</firstterm>. This is a deferred maintenance operation + that deletes index tuples that are known to be safe to delete + (those whose item identifier's <literal>LP_DEAD</literal> bit is + already set). Like bottom-up index deletion, simple index + deletion takes place at the point that a page split is anticipated + as a way of avoiding the split. + </para> + <para> + Simple deletion is opportunistic in the sense that it can only + take place when recent index scans set the + <literal>LP_DEAD</literal> bits of affected items in passing. + Prior to <productname>PostgreSQL</productname> 14, the only + category of B-Tree deletion was simple deletion. The main + differences between it and bottom-up deletion are that only the + former is opportunistically driven by the activity of passing + index scans, while only the latter specifically targets version + churn from <command>UPDATE</command>s that do not logically modify + indexed columns. + </para> + </note> + <para> + Bottom-up index deletion performs the vast majority of all garbage + index tuple cleanup for particular indexes with certain workloads. + This is expected with any B-Tree index that is subject to + significant version churn from <command>UPDATE</command>s that + rarely or never logically modify the columns that the index covers. + The average and worst-case number of versions per logical row can + be kept low purely through targeted incremental deletion passes. + It's quite possible that the on-disk size of certain indexes will + never increase by even one single page/block despite + <emphasis>constant</emphasis> version churn from + <command>UPDATE</command>s. Even then, an exhaustive <quote>clean + sweep</quote> by a <command>VACUUM</command> operation (typically + run in an autovacuum worker process) will eventually be required as + a part of <emphasis>collective</emphasis> cleanup of the table and + each of its indexes. + </para> + <para> + Unlike <command>VACUUM</command>, bottom-up index deletion does not + provide any strong guarantees about how old the oldest garbage + index tuple may be. No index can be permitted to retain + <quote>floating garbage</quote> index tuples that became dead prior + to a conservative cutoff point shared by the table and all of its + indexes collectively. This fundamental table-level invariant makes + it safe to recycle table <acronym>TID</acronym>s. This is how it + is possible for distinct logical rows to reuse the same table + <acronym>TID</acronym> over time (though this can never happen with + two logical rows whose lifetimes span the same + <command>VACUUM</command> cycle). + </para> + </sect2> + + <sect2 id="btree-deduplication"> + <title>Deduplication</title> + <para> + A duplicate is a leaf page tuple (a tuple that points to a table + row) where <emphasis>all</emphasis> indexed key columns have values + that match corresponding column values from at least one other leaf + page tuple in the same index. Duplicate tuples are quite common in + practice. B-Tree indexes can use a special, space-efficient + representation for duplicates when an optional technique is + enabled: <firstterm>deduplication</firstterm>. + </para> + <para> + Deduplication works by periodically merging groups of duplicate + tuples together, forming a single <firstterm>posting list</firstterm> tuple for each + group. The column key value(s) only appear once in this + representation. This is followed by a sorted array of + <acronym>TID</acronym>s that point to rows in the table. This + significantly reduces the storage size of indexes where each value + (or each distinct combination of column values) appears several + times on average. The latency of queries can be reduced + significantly. Overall query throughput may increase + significantly. The overhead of routine index vacuuming may also be + reduced significantly. + </para> + <note> + <para> + B-Tree deduplication is just as effective with + <quote>duplicates</quote> that contain a NULL value, even though + NULL values are never equal to each other according to the + <literal>=</literal> member of any B-Tree operator class. As far + as any part of the implementation that understands the on-disk + B-Tree structure is concerned, NULL is just another value from the + domain of indexed values. + </para> + </note> + <para> + The deduplication process occurs lazily, when a new item is + inserted that cannot fit on an existing leaf page, though only when + index tuple deletion could not free sufficient space for the new + item (typically deletion is briefly considered and then skipped + over). Unlike GIN posting list tuples, B-Tree posting list tuples + do not need to expand every time a new duplicate is inserted; they + are merely an alternative physical representation of the original + logical contents of the leaf page. This design prioritizes + consistent performance with mixed read-write workloads. Most + client applications will at least see a moderate performance + benefit from using deduplication. Deduplication is enabled by + default. + </para> + <para> + <command>CREATE INDEX</command> and <command>REINDEX</command> + apply deduplication to create posting list tuples, though the + strategy they use is slightly different. Each group of duplicate + ordinary tuples encountered in the sorted input taken from the + table is merged into a posting list tuple + <emphasis>before</emphasis> being added to the current pending leaf + page. Individual posting list tuples are packed with as many + <acronym>TID</acronym>s as possible. Leaf pages are written out in + the usual way, without any separate deduplication pass. This + strategy is well-suited to <command>CREATE INDEX</command> and + <command>REINDEX</command> because they are once-off batch + operations. + </para> + <para> + Write-heavy workloads that don't benefit from deduplication due to + having few or no duplicate values in indexes will incur a small, + fixed performance penalty (unless deduplication is explicitly + disabled). The <literal>deduplicate_items</literal> storage + parameter can be used to disable deduplication within individual + indexes. There is never any performance penalty with read-only + workloads, since reading posting list tuples is at least as + efficient as reading the standard tuple representation. Disabling + deduplication isn't usually helpful. + </para> + <para> + It is sometimes possible for unique indexes (as well as unique + constraints) to use deduplication. This allows leaf pages to + temporarily <quote>absorb</quote> extra version churn duplicates. + Deduplication in unique indexes augments bottom-up index deletion, + especially in cases where a long-running transaction holds a + snapshot that blocks garbage collection. The goal is to buy time + for the bottom-up index deletion strategy to become effective + again. Delaying page splits until a single long-running + transaction naturally goes away can allow a bottom-up deletion pass + to succeed where an earlier deletion pass failed. + </para> + <tip> + <para> + A special heuristic is applied to determine whether a + deduplication pass in a unique index should take place. It can + often skip straight to splitting a leaf page, avoiding a + performance penalty from wasting cycles on unhelpful deduplication + passes. If you're concerned about the overhead of deduplication, + consider setting <literal>deduplicate_items = off</literal> + selectively. Leaving deduplication enabled in unique indexes has + little downside. + </para> + </tip> + <para> + Deduplication cannot be used in all cases due to + implementation-level restrictions. Deduplication safety is + determined when <command>CREATE INDEX</command> or + <command>REINDEX</command> is run. + </para> + <para> + Note that deduplication is deemed unsafe and cannot be used in the + following cases involving semantically significant differences + among equal datums: + </para> + <para> + <itemizedlist> + <listitem> + <para> + <type>text</type>, <type>varchar</type>, and <type>char</type> + cannot use deduplication when a + <emphasis>nondeterministic</emphasis> collation is used. Case + and accent differences must be preserved among equal datums. + </para> + </listitem> + + <listitem> + <para> + <type>numeric</type> cannot use deduplication. Numeric display + scale must be preserved among equal datums. + </para> + </listitem> + + <listitem> + <para> + <type>jsonb</type> cannot use deduplication, since the + <type>jsonb</type> B-Tree operator class uses + <type>numeric</type> internally. + </para> + </listitem> + + <listitem> + <para> + <type>float4</type> and <type>float8</type> cannot use + deduplication. These types have distinct representations for + <literal>-0</literal> and <literal>0</literal>, which are + nevertheless considered equal. This difference must be + preserved. + </para> + </listitem> + </itemizedlist> + </para> + <para> + There is one further implementation-level restriction that may be + lifted in a future version of + <productname>PostgreSQL</productname>: + </para> + <para> + <itemizedlist> + <listitem> + <para> + Container types (such as composite types, arrays, or range + types) cannot use deduplication. + </para> + </listitem> + </itemizedlist> + </para> + <para> + There is one further implementation-level restriction that applies + regardless of the operator class or collation used: + </para> + <para> + <itemizedlist> + <listitem> + <para> + <literal>INCLUDE</literal> indexes can never use deduplication. + </para> + </listitem> + </itemizedlist> + </para> + + </sect2> +</sect1> + +</chapter> |