Am 11.08.2015 um 21:03 schrieb Peter Eisentraut:
On 8/10/15 12:36 PM, Peter Moser wrote:
Can someone tell me, how I can compare two datum fields, when I do not
know the data type in advance inside an executor function?
For example, "x less than y" where x and y are of various types that
form intervals. I have found the method ExecTuplesMatch, but it is only
for equality comparison, I think. Another one is ApplySortComparator...
maybe that's the correct way to go?
Some code to make things clearer...
Datum x = heap_getattr(out->tts_tuple,
node->xpos,
out->tts_tupleDescriptor,
&isNull1);
Datum y = slot_getattr(curr, node->ypos, &isNull2);
if (compareDatumWithCorrectMethod(x,y) < 0)
{
/* do something */
}
The tuple descriptor will contain the data type of the datum, so you can
use that to look up the default btree operator class and call the
respective operators in there. But note that there is no single notion
of comparison in the system. Comparison depends on operator class,
access method, possibly collation, null value treatment. Some types
don't support comparison beyond equality. A robust patch would need to
take that into account.
Ok, thank you.
Now I have a first solution. I am just wondering if this is robust, or
do I miss something? Thanks for any comments...
My executor consumes rows from my own rewritten sub-query. From this
sub-query I extract one sortGroupClause and from that the "eqop" and
"sortop" during planning.
sgc = (SortGroupClause *) llast(sortClause);
node->eqOperator = sgc->eqop;
node->ltOperator = sgc->sortop;
The last sort clause uses the same types as the executor needs to
compare later.
The executor initializes the methods with:
state->ltFunctionInfo = (FmgrInfo *) palloc(sizeof(FmgrInfo));
ltOperatorId = get_opcode(node->ltOperator);
fmgr_info(ltOperatorId, state->ltFunctionInfo);
state->eqFunctionInfo = (FmgrInfo *) palloc(sizeof(FmgrInfo));
eqOperatorId = get_opcode(node->eqOperator);
fmgr_info(eqOperatorId, state->eqFunctionInfo);
Finally I use them in this way...
static bool
isLessThan(Datum a, Datum b, FmgrInfo *ltFunctionInfo)
{
return DatumGetBool(FunctionCall2(ltFunctionInfo, a, b));
}
static bool
isEqual(Datum a, Datum b, FmgrInfo *eqFunctionInfo)
{
return DatumGetBool(FunctionCall2(eqFunctionInfo, a, b));
}
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