Sophie: postgresql8.4-docs-8.4.12-0.1mdv2010.2 i586

postgresql8.4-docs-8.4.12-0.1mdv2010.2.i586.rpm

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><A
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>52.5. Limitations</A
></H1
><P
>  <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> doesn't support full index scans.  The reason for
  this is that <CODE
CLASS="FUNCTION"
>extractValue</CODE
> is allowed to return zero keys,
  as for example might happen with an empty string or empty array.  In such
  a case the indexed value will be unrepresented in the index.  It is
  therefore impossible for <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> to guarantee that a
  scan of the index can find every row in the table.
 </P
><P
>  Because of this limitation, when <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> returns
  <TT
CLASS="LITERAL"
>nkeys = 0</TT
> to indicate that all values match the query,
  <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> will emit an error.  (If there are multiple ANDed
  indexable operators in the query, this happens only if they all return zero
  for <TT
CLASS="LITERAL"
>nkeys</TT
>.)
 </P
><P
>  It is possible for an operator class to circumvent the restriction against
  full index scan.  To do that, <CODE
CLASS="FUNCTION"
>extractValue</CODE
> must return at least
  one (possibly dummy) key for every indexed value, and
  <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> must convert an unrestricted search into
  a partial-match query that will scan the whole index.  This is inefficient
  but might be necessary to avoid corner-case failures with operators such
  as <TT
CLASS="LITERAL"
>LIKE</TT
> or subset inclusion.
 </P
><P
>  <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> assumes that indexable operators are strict.
  This means that <CODE
CLASS="FUNCTION"
>extractValue</CODE
> will not be called at all on
  a NULL value (so the value will go unindexed), and
  <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> will not be called on a NULL comparison
  value either (instead, the query is presumed to be unmatchable).
 </P
><P
>  A possibly more serious limitation is that <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> cannot
  handle NULL keys &mdash; for example, an array containing a NULL cannot
  be handled except by ignoring the NULL.
 </P
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