Fix incorrect column number in funcTupleDesc initialization.

In process_sample_rows(), when initializing funcTupleDesc for table columns,
the column number should start from NUM_SAMPLE_FIXED_COLS + 1 (5) instead of 4.

The first 4 columns (1-4) are reserved for fixed columns:
- Column 1: totalrows (FLOAT8OID)
- Column 2: totaldeadrows (FLOAT8OID)
- Column 3: oversized_cols_length (FLOAT8ARRAYOID)
- Column 4: NDV array (FLOAT8ARRAYOID)

Table columns should start from column 5, so the correct formula is:
(AttrNumber) NUM_SAMPLE_FIXED_COLS + 1 + index

This bug is harmless because funcTupleDesc's column type information is not
actually used in subsequent processing - only the column count (natts) is used.
The actual type information is obtained dynamically via lookup_rowtype_tupdesc().
However, it's still worth fixing for code correctness and maintainability.

Author: zhangyue-hashdata

src/backend/commands/analyze.c

@@ -2758,7 +2758,7 @@ process_sample_rows(Portal portal,
 
 		if (!attr->attisdropped)
 		{
-			TupleDescInitEntry(funcTupleDesc, (AttrNumber) 4 + index, "",
+			TupleDescInitEntry(funcTupleDesc, (AttrNumber) NUM_SAMPLE_FIXED_COLS + 1 + index, "",
 							   typid, attr->atttypmod, attr->attndims);
 
 			index++;