145 lines
5.2 KiB
HTML
145 lines
5.2 KiB
HTML
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<title>Obtain Aggregate Function Context</title>
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Obtain Aggregate Function Context</h2>
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</div>
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<blockquote><pre>
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void *sqlite3_aggregate_context(sqlite3_context*, int nBytes);
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</pre></blockquote>
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<p>
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Implementations of aggregate SQL functions use this
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routine to allocate memory for storing their state.</p>
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<p>The first time the sqlite3_aggregate_context(C,N) routine is called
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for a particular aggregate function, SQLite allocates
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N bytes of memory, zeroes out that memory, and returns a pointer
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to the new memory. On second and subsequent calls to
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sqlite3_aggregate_context() for the same aggregate function instance,
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the same buffer is returned. Sqlite3_aggregate_context() is normally
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called once for each invocation of the xStep callback and then one
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last time when the xFinal callback is invoked. When no rows match
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an aggregate query, the xStep() callback of the aggregate function
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implementation is never called and xFinal() is called exactly once.
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In those cases, sqlite3_aggregate_context() might be called for the
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first time from within xFinal().</p>
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<p>The sqlite3_aggregate_context(C,N) routine returns a NULL pointer
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when first called if N is less than or equal to zero or if a memory
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allocate error occurs.</p>
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<p>The amount of space allocated by sqlite3_aggregate_context(C,N) is
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determined by the N parameter on first successful call. Changing the
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value of N in any subsequent call to sqlite3_aggregate_context() within
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the same aggregate function instance will not resize the memory
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allocation. Within the xFinal callback, it is customary to set
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N=0 in calls to sqlite3_aggregate_context(C,N) so that no
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pointless memory allocations occur.</p>
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<p>SQLite automatically frees the memory allocated by
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sqlite3_aggregate_context() when the aggregate query concludes.</p>
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<p>The first parameter must be a copy of the
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<a href="../c3ref/context.html">SQL function context</a> that is the first parameter
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to the xStep or xFinal callback routine that implements the aggregate
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function.</p>
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<p>This routine must be called from the same thread in which
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the aggregate SQL function is running.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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