[HACKERS] Cstring vs. Datum values ( BuildTupleFromCStrings vs.BlessTupleDesc)
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From: Ivan Sergio Borgonovo on 23 Jan 2010 07:56 Hi, I'm trying to write a couple of C function that will: - return a tsvector as a set of record. - turn a tsvector into a tsquery I'm looking at the example function to return set of records http://www.postgresql.org/docs/8.4/static/xfunc-c.html#AEN44970 and to this: "There are two ways you can build a composite data value (henceforth a "tuple"): you can build it from an array of Datum values, or from an array of C strings that can be passed to the input conversion functions of the tuple's column data types. In either case, you first need to obtain or construct a TupleDesc descriptor for the tuple structure. When working with Datums, you pass the TupleDesc to BlessTupleDesc, and then call heap_form_tuple for each row. When working with C strings, you pass the TupleDesc to TupleDescGetAttInMetadata, and then call BuildTupleFromCStrings for each row. In the case of a function returning a set of tuples, the setup steps can all be done once during the first call of the function." And I can't understand if I can avoid transforming everything into its text representation: snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1)); snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1)); snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1)); And if I can... how, why and when... because I didn't find any clear example in the source tree that gives me a clue about when I'd use one form or the other. Up to my understanding the way to go to use Datum is: /* create a template tupledesc specifying numbers of columns (eg.2) */ tupdesc = CreateTemplateTupleDesc(3, false); /* ??? bool hasoid */ /* configure the "fields" */ TupleDescInitEntry(tupdesc, (AttrNumber) 1, "word", TEXTOID, -1, 0); /* ?????? Oid oidtypeid, // where can I get a list of OID int32 typmod, int attdim) */ /* ??? */ funcctx->tuple_desc = BlessTupleDesc(tupdesc); funcctx->attinmeta = TupleDescGetAttInMetadata(tupdesc); -- Ivan Sergio Borgonovo http://www.webthatworks.it -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers |