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>Chapter 52. Index Access Method Interface Definition</H1
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>Table of Contents</B
></DT
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>52.1. <A
HREF="index-catalog.html"
>Catalog Entries for Indexes</A
></DT
><DT
>52.2. <A
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>Index Access Method Functions</A
></DT
><DT
>52.3. <A
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>Index Scanning</A
></DT
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>52.4. <A
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>Index Locking Considerations</A
></DT
><DT
>52.5. <A
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>Index Uniqueness Checks</A
></DT
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>52.6. <A
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>Index Cost Estimation Functions</A
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> This chapter defines the interface between the core
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> system and <I
CLASS="FIRSTTERM"
>index access
methods</I
>, which manage individual index types. The core system
knows nothing about indexes beyond what is specified here, so it is
possible to develop entirely new index types by writing add-on code.
</P
><P
> All indexes in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> are what are known
technically as <I
CLASS="FIRSTTERM"
>secondary indexes</I
>; that is, the index is
physically separate from the table file that it describes. Each index
is stored as its own physical <I
CLASS="FIRSTTERM"
>relation</I
> and so is described
by an entry in the <TT
CLASS="STRUCTNAME"
>pg_class</TT
> catalog. The contents of an
index are entirely under the control of its index access method. In
practice, all index access methods divide indexes into standard-size
pages so that they can use the regular storage manager and buffer manager
to access the index contents. (All the existing index access methods
furthermore use the standard page layout described in <A
HREF="storage-page-layout.html"
>Section 56.6</A
>, and they all use the same format for index
tuple headers; but these decisions are not forced on an access method.)
</P
><P
> An index is effectively a mapping from some data key values to
<I
CLASS="FIRSTTERM"
>tuple identifiers</I
>, or <ACRONYM
CLASS="ACRONYM"
>TIDs</ACRONYM
>, of row versions
(tuples) in the index's parent table. A TID consists of a
block number and an item number within that block (see <A
HREF="storage-page-layout.html"
>Section 56.6</A
>). This is sufficient
information to fetch a particular row version from the table.
Indexes are not directly aware that under MVCC, there might be multiple
extant versions of the same logical row; to an index, each tuple is
an independent object that needs its own index entry. Thus, an
update of a row always creates all-new index entries for the row, even if
the key values did not change. (HOT tuples are an exception to this
statement; but indexes do not deal with those, either.) Index entries for
dead tuples are reclaimed (by vacuuming) when the dead tuples themselves
are reclaimed.
</P
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