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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="GIN-EXTENSIBILITY"
>55.2. Extensibility</A
></H1
><P
> The <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> interface has a high level of abstraction,
requiring the access method implementer only to implement the semantics of
the data type being accessed. The <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> layer itself
takes care of concurrency, logging and searching the tree structure.
</P
><P
> All it takes to get a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> access method working is to
implement four (or five) user-defined methods, which define the behavior of
keys in the tree and the relationships between keys, indexed items,
and indexable queries. In short, <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> combines
extensibility with generality, code reuse, and a clean interface.
</P
><P
> The four methods that an operator class for
<ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> must provide are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><CODE
CLASS="FUNCTION"
>int compare(Datum a, Datum b)</CODE
></DT
><DD
><P
> Compares two keys (not indexed items!) and returns an integer less than
zero, zero, or greater than zero, indicating whether the first key is
less than, equal to, or greater than the second. Null keys are never
passed to this function.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>Datum *extractValue(Datum itemValue, int32 *nkeys,
bool **nullFlags)</CODE
></DT
><DD
><P
> Returns a palloc'd array of keys given an item to be indexed. The
number of returned keys must be stored into <TT
CLASS="LITERAL"
>*nkeys</TT
>.
If any of the keys can be null, also palloc an array of
<TT
CLASS="LITERAL"
>*nkeys</TT
> booleans, store its address at
<TT
CLASS="LITERAL"
>*nullFlags</TT
>, and set these null flags as needed.
<TT
CLASS="LITERAL"
>*nullFlags</TT
> can be left <TT
CLASS="SYMBOL"
>NULL</TT
> (its initial value)
if all keys are non-null.
The return value can be <TT
CLASS="SYMBOL"
>NULL</TT
> if the item contains no keys.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>Datum *extractQuery(Datum query, int32 *nkeys,
StrategyNumber n, bool **pmatch, Pointer **extra_data,
bool **nullFlags, int32 *searchMode)</CODE
></DT
><DD
><P
> Returns a palloc'd array of keys given a value to be queried; that is,
<TT
CLASS="LITERAL"
>query</TT
> is the value on the right-hand side of an
indexable operator whose left-hand side is the indexed column.
<TT
CLASS="LITERAL"
>n</TT
> is the strategy number of the operator within the
operator class (see <A
HREF="xindex.html#XINDEX-STRATEGIES"
>Section 35.14.2</A
>).
Often, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> will need
to consult <TT
CLASS="LITERAL"
>n</TT
> to determine the data type of
<TT
CLASS="LITERAL"
>query</TT
> and the method it should use to extract key values.
The number of returned keys must be stored into <TT
CLASS="LITERAL"
>*nkeys</TT
>.
If any of the keys can be null, also palloc an array of
<TT
CLASS="LITERAL"
>*nkeys</TT
> booleans, store its address at
<TT
CLASS="LITERAL"
>*nullFlags</TT
>, and set these null flags as needed.
<TT
CLASS="LITERAL"
>*nullFlags</TT
> can be left NULL (its initial value)
if all keys are non-null.
The return value can be NULL if the <TT
CLASS="LITERAL"
>query</TT
> contains no keys.
</P
><P
> <TT
CLASS="LITERAL"
>searchMode</TT
> is an output argument that allows
<CODE
CLASS="FUNCTION"
>extractQuery</CODE
> to specify details about how the search
will be done.
If <TT
CLASS="LITERAL"
>*searchMode</TT
> is set to
<TT
CLASS="LITERAL"
>GIN_SEARCH_MODE_DEFAULT</TT
> (which is the value it is
initialized to before call), only items that match at least one of
the returned keys are considered candidate matches.
If <TT
CLASS="LITERAL"
>*searchMode</TT
> is set to
<TT
CLASS="LITERAL"
>GIN_SEARCH_MODE_INCLUDE_EMPTY</TT
>, then in addition to items
containing at least one matching key, items that contain no keys at
all are considered candidate matches. (This mode is useful for
implementing is-subset-of operators, for example.)
If <TT
CLASS="LITERAL"
>*searchMode</TT
> is set to <TT
CLASS="LITERAL"
>GIN_SEARCH_MODE_ALL</TT
>,
then all non-null items in the index are considered candidate
matches, whether they match any of the returned keys or not. (This
mode is much slower than the other two choices, since it requires
scanning essentially the entire index, but it may be necessary to
implement corner cases correctly. An operator that needs this mode
in most cases is probably not a good candidate for a GIN operator
class.)
The symbols to use for setting this mode are defined in
<TT
CLASS="FILENAME"
>access/gin.h</TT
>.
</P
><P
> <TT
CLASS="LITERAL"
>pmatch</TT
> is an output argument for use when partial match
is supported. To use it, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> must allocate
an array of <TT
CLASS="LITERAL"
>*nkeys</TT
> booleans and store its address at
<TT
CLASS="LITERAL"
>*pmatch</TT
>. Each element of the array should be set to TRUE
if the corresponding key requires partial match, FALSE if not.
If <TT
CLASS="LITERAL"
>*pmatch</TT
> is set to NULL then GIN assumes partial match
is not required. The variable is initialized to NULL before call,
so this argument can simply be ignored by operator classes that do
not support partial match.
</P
><P
> <TT
CLASS="LITERAL"
>extra_data</TT
> is an output argument that allows
<CODE
CLASS="FUNCTION"
>extractQuery</CODE
> to pass additional data to the
<CODE
CLASS="FUNCTION"
>consistent</CODE
> and <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> methods.
To use it, <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> must allocate
an array of <TT
CLASS="LITERAL"
>*nkeys</TT
> pointers and store its address at
<TT
CLASS="LITERAL"
>*extra_data</TT
>, then store whatever it wants to into the
individual pointers. The variable is initialized to NULL before
call, so this argument can simply be ignored by operator classes that
do not require extra data. If <TT
CLASS="LITERAL"
>*extra_data</TT
> is set, the
whole array is passed to the <CODE
CLASS="FUNCTION"
>consistent</CODE
> method, and
the appropriate element to the <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> method.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>bool consistent(bool check[], StrategyNumber n, Datum query,
int32 nkeys, Pointer extra_data[], bool *recheck,
Datum queryKeys[], bool nullFlags[])</CODE
></DT
><DD
><P
> Returns TRUE if an indexed item satisfies the query operator with
strategy number <TT
CLASS="LITERAL"
>n</TT
> (or might satisfy it, if the recheck
indication is returned). This function does not have direct access
to the indexed item's value, since <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> does not
store items explicitly. Rather, what is available is knowledge
about which key values extracted from the query appear in a given
indexed item. The <TT
CLASS="LITERAL"
>check</TT
> array has length
<TT
CLASS="LITERAL"
>nkeys</TT
>, which is the same as the number of keys previously
returned by <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> for this <TT
CLASS="LITERAL"
>query</TT
> datum.
Each element of the
<TT
CLASS="LITERAL"
>check</TT
> array is TRUE if the indexed item contains the
corresponding query key, ie, if (check[i] == TRUE) the i-th key of the
<CODE
CLASS="FUNCTION"
>extractQuery</CODE
> result array is present in the indexed item.
The original <TT
CLASS="LITERAL"
>query</TT
> datum is
passed in case the <CODE
CLASS="FUNCTION"
>consistent</CODE
> method needs to consult it,
and so are the <TT
CLASS="LITERAL"
>queryKeys[]</TT
> and <TT
CLASS="LITERAL"
>nullFlags[]</TT
>
arrays previously returned by <CODE
CLASS="FUNCTION"
>extractQuery</CODE
>.
<TT
CLASS="LITERAL"
>extra_data</TT
> is the extra-data array returned by
<CODE
CLASS="FUNCTION"
>extractQuery</CODE
>, or NULL if none.
</P
><P
> When <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> returns a null key in
<TT
CLASS="LITERAL"
>queryKeys[]</TT
>, the corresponding <TT
CLASS="LITERAL"
>check[]</TT
> element
is TRUE if the indexed item contains a null key; that is, the
semantics of <TT
CLASS="LITERAL"
>check[]</TT
> are like <TT
CLASS="LITERAL"
>IS NOT DISTINCT
FROM</TT
>. The <CODE
CLASS="FUNCTION"
>consistent</CODE
> function can examine the
corresponding <TT
CLASS="LITERAL"
>nullFlags[]</TT
> element if it needs to tell
the difference between a regular value match and a null match.
</P
><P
> On success, <TT
CLASS="LITERAL"
>*recheck</TT
> should be set to TRUE if the heap
tuple needs to be rechecked against the query operator, or FALSE if
the index test is exact. That is, a FALSE return value guarantees
that the heap tuple does not match the query; a TRUE return value with
<TT
CLASS="LITERAL"
>*recheck</TT
> set to FALSE guarantees that the heap tuple does
match the query; and a TRUE return value with
<TT
CLASS="LITERAL"
>*recheck</TT
> set to TRUE means that the heap tuple might match
the query, so it needs to be fetched and rechecked by evaluating the
query operator directly against the originally indexed item.
</P
></DD
></DL
></DIV
><P>
Optionally, an operator class for
<ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> can supply a fifth method:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><CODE
CLASS="FUNCTION"
>int comparePartial(Datum partial_key, Datum key, StrategyNumber n,
Pointer extra_data)</CODE
></DT
><DD
><P
> Compare a partial-match query key to an index key. Returns an integer
whose sign indicates the result: less than zero means the index key
does not match the query, but the index scan should continue; zero
means that the index key does match the query; greater than zero
indicates that the index scan should stop because no more matches
are possible. The strategy number <TT
CLASS="LITERAL"
>n</TT
> of the operator
that generated the partial match query is provided, in case its
semantics are needed to determine when to end the scan. Also,
<TT
CLASS="LITERAL"
>extra_data</TT
> is the corresponding element of the extra-data
array made by <CODE
CLASS="FUNCTION"
>extractQuery</CODE
>, or NULL if none.
Null keys are never passed to this function.
</P
></DD
></DL
></DIV
><P>
</P
><P
> To support <SPAN
CLASS="QUOTE"
>"partial match"</SPAN
> queries, an operator class must
provide the <CODE
CLASS="FUNCTION"
>comparePartial</CODE
> method, and its
<CODE
CLASS="FUNCTION"
>extractQuery</CODE
> method must set the <TT
CLASS="LITERAL"
>pmatch</TT
>
parameter when a partial-match query is encountered. See
<A
HREF="gin-implementation.html#GIN-PARTIAL-MATCH"
>Section 55.3.2</A
> for details.
</P
><P
> The actual data types of the various <TT
CLASS="LITERAL"
>Datum</TT
> values mentioned
above vary depending on the operator class. The item values passed to
<CODE
CLASS="FUNCTION"
>extractValue</CODE
> are always of the operator class's input type, and
all key values must be of the class's <TT
CLASS="LITERAL"
>STORAGE</TT
> type. The type of
the <TT
CLASS="LITERAL"
>query</TT
> argument passed to <CODE
CLASS="FUNCTION"
>extractQuery</CODE
> and
<CODE
CLASS="FUNCTION"
>consistent</CODE
> is whatever is specified as the right-hand input
type of the class member operator identified by the strategy number.
This need not be the same as the item type, so long as key values of the
correct type can be extracted from it.
</P
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