src/sgml/btree.sgml

<!-- 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>&lt;</literal>,
  <literal>&lt;=</literal>,
  <literal>=</literal>,
  <literal>&gt;=</literal> and
  <literal>&gt;</literal>.
  One might expect that <literal>&lt;&gt;</literal> should also be part of
  the operator class, but it is not, because it would almost never be
  useful to use a <literal>&lt;&gt;</literal> WHERE clause in an index
  search.  (For some purposes, the planner treats <literal>&lt;&gt;</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>&lt;</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>&lt;</literal>
       <replaceable>A</replaceable> is false
       (<firstterm>irreflexive law</firstterm>)
      </para>
     </listitem>
     <listitem>
      <para>
       if <replaceable>A</replaceable> <literal>&lt;</literal>
       <replaceable>B</replaceable>
       and <replaceable>B</replaceable> <literal>&lt;</literal>
       <replaceable>C</replaceable>,
       then <replaceable>A</replaceable> <literal>&lt;</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>&lt;</literal>
       <replaceable>B</replaceable>, <replaceable>A</replaceable>
       <literal>=</literal> <replaceable>B</replaceable>, and
       <replaceable>B</replaceable> <literal>&lt;</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>&lt;</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>&lt;</literal> <literal>0</literal>,
     <literal>0</literal>, or <literal>&gt;</literal>
     <literal>0</literal> when <replaceable>A</replaceable>
     <literal>&lt;</literal> <replaceable>B</replaceable>,
     <replaceable>A</replaceable> <literal>=</literal>
     <replaceable>B</replaceable>, or <replaceable>A</replaceable>
     <literal>&gt;</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>&gt;=</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>&lt;=</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>&gt;=</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>&lt;=</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>&lt;=</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>&gt;=</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>&gt;=</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>&lt;=</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 <replaceable>local_relopts</replaceable>
     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-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.  This prevents
   (or at least delays) leaf page splits.  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>
   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 duplicates</quote> 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 <acronym>HOT</acronym>
   optimization (often because at least one indexed column gets
   modified, necessitating a new set of index tuple versions &mdash;
   one new tuple for <emphasis>each and every</emphasis> index).  In
   effect, B-Tree deduplication ameliorates index bloat caused by
   version churn.  Note that even the tuples from a unique index are
   not necessarily <emphasis>physically</emphasis> unique when stored
   on disk due to version churn.  The deduplication optimization is
   selectively applied within unique indexes.  It targets those pages
   that appear to have version duplicates.  The high level goal is to
   give <command>VACUUM</command> more time to run before an
   <quote>unnecessary</quote> page split caused by version churn can
   take place.
  </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>