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><H1
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><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
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><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
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><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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