The way that starting and stopping keys and operators are used may best be described by example. Say there's an ordered conglomerate with two columns, where the 0-th column is named 'x', and the 1st column is named 'y'. The values of the columns are as follows:
x: 1 3 4 4 4 5 5 5 6 7 9 y: 1 1 2 4 6 2 4 6 1 1 1
A {start key, search op} pair of {{5.2}, GE} would position on {x=5, y=2}, whereas the pair {{5}, GT} would position on {x=6, y=1}.
Partial keys are used to implement partial key scans in SQL. For example, the SQL "select * from t where x = 5" would open a scan on the conglomerate (or a useful index) of t using a starting position partial key of {{5}, GE} and a stopping position partial key of {{5}, GT}.
Some more examples:
+-------------------+------------+-----------+--------------+--------------+ | predicate | start key | stop key | rows | rows locked | | | value | op | value |op | returned |serialization | +-------------------+-------+----+-------+---+--------------+--------------+ | x = 5 | {5} | GE | {5} |GT |{5,2} .. {5,6}|{4,6} .. {5,6}| | x > 5 | {5} | GT | null | |{6,1} .. {9,1}|{5,6} .. {9,1}| | x >= 5 | {5} | GE | null | |{5,2} .. {9,1}|{4,6} .. {9,1}| | x <= 5 | null | | {5} |GT |{1,1} .. {5,6}|first .. {5,6}| | x < 5 | null | | {5} |GE |{1,1} .. {4,6}|first .. {4,6}| | x >= 5 and x <= 7 | {5}, | GE | {7} |GT |{5,2} .. {7,1}|{4,6} .. {7,1}| | x = 5 and y > 2 | {5,2} | GT | {5} |GT |{5,4} .. {5,6}|{5,2} .. {5,6}| | x = 5 and y >= 2 | {5,2} | GE | {5} |GT |{5,2} .. {5,6}|{4,6} .. {5,6}| | x = 5 and y < 5 | {5} | GE | {5,5} |GE |{5,2} .. {5,4}|{4,6} .. {5,4}| | x = 2 | {2} | GE | {2} |GT | none |{1,1} .. {1,1}| +-------------------+-------+----+-------+---+--------------+--------------+ As the above table implies, the underlying scan may lock more rows than it returns in order to guarantee serialization.
For each row which meets the start and stop position, as described above the row is "qualified" to see whether it should be returned. The qualification is a 2 dimensional array of @see Qualifiers, which represents the qualification in conjunctive normal form (CNF). Conjunctive normal form is an "and'd" set of "or'd" Qualifiers.
For example x = 5 would be represented is pseudo code as: qualifier_cnf[][] = new Qualifier[1]; qualifier_cnf[0] = new Qualifier[1]; qualifier_cnr[0][0] = new Qualifer(x = 5)
For example (x = 5) or (y = 6) would be represented is pseudo code as: qualifier_cnf[][] = new Qualifier[1]; qualifier_cnf[0] = new Qualifier[2]; qualifier_cnr[0][0] = new Qualifer(x = 5) qualifier_cnr[0][1] = new Qualifer(y = 6)
For example ((x = 5) or (x = 6)) and ((y = 1) or (y = 2)) would be represented is pseudo code as: qualifier_cnf[][] = new Qualifier[2]; qualifier_cnf[0] = new Qualifier[2]; qualifier_cnr[0][0] = new Qualifer(x = 5) qualifier_cnr[0][1] = new Qualifer(x = 6) qualifier_cnr[0][0] = new Qualifer(y = 5) qualifier_cnr[0][1] = new Qualifer(y = 6)
For each row the CNF qualfier is processed and it is determined whether or not the row should be returned to the caller. The following pseudo-code describes how this is done:
if (qualifier != null) { for (int and_clause; and_clause < qualifier.length; and_clause++) { boolean or_qualifies = false; for (int or_clause; or_clause < qualifier[and_clause].length; or_clause++) { DataValueDescriptor key = qualifier[and_clause][or_clause].getOrderable(); DataValueDescriptor row_col = get row column[qualifier[and_clause][or_clause].getColumnId()]; boolean or_qualifies = row_col.compare(qualifier[i].getOperator, key, qualifier[i].getOrderedNulls, qualifier[i].getUnknownRV);
if (or_qualifies) { break; } } if (!or_qualifies) { don't return this row to the client - proceed to next row;
}
} | |
| |
| |
| |
| |
| |
| |
| |
| |
| |