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><H1
CLASS="SECT1"
><A
NAME="GIN-IMPLEMENTATION"
>55.3. Implementation</A
></H1
><P
>  Internally, a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index contains a B-tree index
  constructed over keys, where each key is an element of one or more indexed
  items (a member of an array, for example) and where each tuple in a leaf
  page contains either a pointer to a B-tree of heap pointers (a
  <SPAN
CLASS="QUOTE"
>"posting tree"</SPAN
>), or a simple list of heap pointers (a <SPAN
CLASS="QUOTE"
>"posting
  list"</SPAN
>) when the list is small enough to fit into a single index tuple along
  with the key value.
 </P
><P
>  As of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.1, NULL key values can be
  included in the index.  Also, placeholder NULLs are included in the index
  for indexed items that are NULL or contain no keys according to
  <CODE
CLASS="FUNCTION"
>extractValue</CODE
>.  This allows searches that should find empty
  items to do so.
 </P
><P
>  Multicolumn <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> indexes are implemented by building
  a single B-tree over composite values (column number, key value).  The
  key values for different columns can be of different types.
 </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="GIN-FAST-UPDATE"
>55.3.1. GIN Fast Update Technique</A
></H2
><P
>   Updating a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index tends to be slow because of the
   intrinsic nature of inverted indexes: inserting or updating one heap row
   can cause many inserts into the index (one for each key extracted
   from the indexed item). As of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.4,
   <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> is capable of postponing much of this work by inserting
   new tuples into a temporary, unsorted list of pending entries.
   When the table is vacuumed, or if the pending list becomes too large
   (larger than <A
HREF="runtime-config-resource.html#GUC-WORK-MEM"
>work_mem</A
>), the entries are moved to the
   main <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> data structure using the same bulk insert
   techniques used during initial index creation.  This greatly improves
   <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index update speed, even counting the additional
   vacuum overhead.  Moreover the overhead work can be done by a background
   process instead of in foreground query processing.
  </P
><P
>   The main disadvantage of this approach is that searches must scan the list
   of pending entries in addition to searching the regular index, and so
   a large list of pending entries will slow searches significantly.
   Another disadvantage is that, while most updates are fast, an update
   that causes the pending list to become <SPAN
CLASS="QUOTE"
>"too large"</SPAN
> will incur an
   immediate cleanup cycle and thus be much slower than other updates.
   Proper use of autovacuum can minimize both of these problems.
  </P
><P
>   If consistent response time is more important than update speed,
   use of pending entries can be disabled by turning off the
   <TT
CLASS="LITERAL"
>FASTUPDATE</TT
> storage parameter for a
   <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index.  See <A
HREF="sql-createindex.html"
>CREATE INDEX</A
>
   for details.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="GIN-PARTIAL-MATCH"
>55.3.2. Partial Match Algorithm</A
></H2
><P
>   GIN can support <SPAN
CLASS="QUOTE"
>"partial match"</SPAN
> queries, in which the query
   does not determine an exact match for one or more keys, but the possible
   matches fall within a reasonably narrow range of key values (within the
   key sorting order determined by the <CODE
CLASS="FUNCTION"
>compare</CODE
> support method).
   The <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> method, instead of returning a key value
   to be matched exactly, returns a key value that is the lower bound of
   the range to be searched, and sets the <TT
CLASS="LITERAL"
>pmatch</TT
> flag true.
   The key range is then scanned using the <CODE
CLASS="FUNCTION"
>comparePartial</CODE
>
   method.  <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> must return zero for a matching
   index key, less than zero for a non-match that is still within the range
   to be searched, or greater than zero if the index key is past the range
   that could match.
  </P
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