| PostgreSQL 7.4.8 Documentation | ||||
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A PostgreSQL operator definition can include several optional clauses that tell the system useful things about how the operator behaves. These clauses should be provided whenever appropriate, because they can make for considerable speedups in execution of queries that use the operator. But if you provide them, you must be sure that they are right! Incorrect use of an optimization clause can result in server process crashes, subtly wrong output, or other Bad Things. You can always leave out an optimization clause if you are not sure about it; the only consequence is that queries might run slower than they need to.
Additional optimization clauses might be added in future versions of PostgreSQL. The ones described here are all the ones that release 7.4.8 understands.
The COMMUTATOR clause, if provided, names an operator that is the commutator of the operator being defined. We say that operator A is the commutator of operator B if (x A y) equals (y B x) for all possible input values x, y. Notice that B is also the commutator of A. For example, operators < and > for a particular data type are usually each others' commutators, and operator + is usually commutative with itself. But operator - is usually not commutative with anything.
The left operand type of a commutable operator is the same as the right operand type of its commutator, and vice versa. So the name of the commutator operator is all that PostgreSQL needs to be given to look up the commutator, and that's all that needs to be provided in the COMMUTATOR clause.
It's critical to provide commutator information for operators that will be used in indexes and join clauses, because this allows the query optimizer to "flip around" such a clause to the forms needed for different plan types. For example, consider a query with a WHERE clause like tab1.x = tab2.y, where tab1.x and tab2.y are of a user-defined type, and suppose that tab2.y is indexed. The optimizer cannot generate an index scan unless it can determine how to flip the clause around to tab2.y = tab1.x, because the index-scan machinery expects to see the indexed column on the left of the operator it is given. PostgreSQL will not simply assume that this is a valid transformation --- the creator of the = operator must specify that it is valid, by marking the operator with commutator information.
When you are defining a self-commutative operator, you just do it. When you are defining a pair of commutative operators, things are a little trickier: how can the first one to be defined refer to the other one, which you haven't defined yet? There are two solutions to this problem:
One way is to omit the COMMUTATOR clause in the first operator that you define, and then provide one in the second operator's definition. Since PostgreSQL knows that commutative operators come in pairs, when it sees the second definition it will automatically go back and fill in the missing COMMUTATOR clause in the first definition.
The other, more straightforward way is just to include COMMUTATOR clauses in both definitions. When PostgreSQL processes the first definition and realizes that COMMUTATOR refers to a nonexistent operator, the system will make a dummy entry for that operator in the system catalog. This dummy entry will have valid data only for the operator name, left and right operand types, and result type, since that's all that PostgreSQL can deduce at this point. The first operator's catalog entry will link to this dummy entry. Later, when you define the second operator, the system updates the dummy entry with the additional information from the second definition. If you try to use the dummy operator before it's been filled in, you'll just get an error message.
The NEGATOR clause, if provided, names an operator that is the negator of the operator being defined. We say that operator A is the negator of operator B if both return Boolean results and (x A y) equals NOT (x B y) for all possible inputs x, y. Notice that B is also the negator of A. For example, < and >= are a negator pair for most data types. An operator can never validly be its own negator.
Unlike commutators, a pair of unary operators could validly be marked as each others' negators; that would mean (A x) equals NOT (B x) for all x, or the equivalent for right unary operators.
An operator's negator must have the same left and/or right operand types as the operator to be defined, so just as with COMMUTATOR, only the operator name need be given in the NEGATOR clause.
Providing a negator is very helpful to the query optimizer since it allows expressions like NOT (x = y) to be simplified into x <> y. This comes up more often than you might think, because NOT operations can be inserted as a consequence of other rearrangements.
Pairs of negator operators can be defined using the same methods explained above for commutator pairs.
The RESTRICT clause, if provided, names a restriction selectivity estimation function for the operator. (Note that this is a function name, not an operator name.) RESTRICT clauses only make sense for binary operators that return boolean. The idea behind a restriction selectivity estimator is to guess what fraction of the rows in a table will satisfy a WHERE-clause condition of the form
column OP constant
for the current operator and a particular constant value. This assists the optimizer by giving it some idea of how many rows will be eliminated by WHERE clauses that have this form. (What happens if the constant is on the left, you may be wondering? Well, that's one of the things that COMMUTATOR is for...)
Writing new restriction selectivity estimation functions is far beyond the scope of this chapter, but fortunately you can usually just use one of the system's standard estimators for many of your own operators. These are the standard restriction estimators:
eqsel for = |
neqsel for <> |
scalarltsel for < or <= |
scalargtsel for > or >= |
You can frequently get away with using either eqsel or neqsel for
operators that have very high or very low selectivity, even if they
aren't really equality or inequality. For example, the
approximate-equality geometric operators use eqsel on the assumption that
they'll usually only match a small fraction of the entries in a table.
You can use scalarltsel and scalargtsel for comparisons on data types that
have some sensible means of being converted into numeric scalars for
range comparisons. If possible, add the data type to those understood
by the function convert_to_scalar() in src/backend/utils/adt/selfuncs.c.
(Eventually, this function should be replaced by per-data-type functions
identified through a column of the pg_type system catalog; but that hasn't happened
yet.) If you do not do this, things will still work, but the optimizer's
estimates won't be as good as they could be.
There are additional selectivity estimation functions designed for geometric
operators in src/backend/utils/adt/geo_selfuncs.c: areasel, positionsel,
and contsel. At this writing these are just stubs, but you may want
to use them (or even better, improve them) anyway.
The JOIN clause, if provided, names a join selectivity estimation function for the operator. (Note that this is a function name, not an operator name.) JOIN clauses only make sense for binary operators that return boolean. The idea behind a join selectivity estimator is to guess what fraction of the rows in a pair of tables will satisfy a WHERE-clause condition of the form
table1.column1 OP table2.column2
for the current operator. As with the RESTRICT clause, this helps the optimizer very substantially by letting it figure out which of several possible join sequences is likely to take the least work.
As before, this chapter will make no attempt to explain how to write a join selectivity estimator function, but will just suggest that you use one of the standard estimators if one is applicable:
eqjoinsel for = |
neqjoinsel for <> |
scalarltjoinsel for < or <= |
scalargtjoinsel for > or >= |
areajoinsel for 2D area-based comparisons |
positionjoinsel for 2D position-based comparisons |
contjoinsel for 2D containment-based comparisons |
The HASHES clause, if present, tells the system that it is permissible to use the hash join method for a join based on this operator. HASHES only makes sense for a binary operator that returns boolean, and in practice the operator had better be equality for some data type.
The assumption underlying hash join is that the join operator can only return true for pairs of left and right values that hash to the same hash code. If two values get put in different hash buckets, the join will never compare them at all, implicitly assuming that the result of the join operator must be false. So it never makes sense to specify HASHES for operators that do not represent equality.
To be marked HASHES, the join operator must appear in a hash index operator class. This is not enforced when you create the operator, since of course the referencing operator class couldn't exist yet. But attempts to use the operator in hash joins will fail at runtime if no such operator class exists. The system needs the operator class to find the data-type-specific hash function for the operator's input data type. Of course, you must also supply a suitable hash function before you can create the operator class.
Care should be exercised when preparing a hash function, because there
are machine-dependent ways in which it might fail to do the right thing.
For example, if your data type is a structure in which there may be
uninteresting pad bits, you can't simply pass the whole structure to
hash_any. (Unless you write your other operators and
functions to ensure that the unused bits are always zero, which is the
recommended strategy.)
Another example is that on machines that meet the IEEE
floating-point standard, negative zero and positive zero are different
values (different bit patterns) but they are defined to compare equal.
If a float value might contain negative zero then extra steps are needed
to ensure it generates the same hash value as positive zero.
Note: The function underlying a hash-joinable operator must be marked immutable or stable. If it is volatile, the system will never attempt to use the operator for a hash join.
Note: If a hash-joinable operator has an underlying function that is marked strict, the function must also be complete: that is, it should return true or false, never null, for any two nonnull inputs. If this rule is not followed, hash-optimization of IN operations may generate wrong results. (Specifically, IN might return false where the correct answer according to the standard would be null; or it might yield an error complaining that it wasn't prepared for a null result.)
The MERGES clause, if present, tells the system that it is permissible to use the merge-join method for a join based on this operator. MERGES only makes sense for a binary operator that returns boolean, and in practice the operator must represent equality for some data type or pair of data types.
Merge join is based on the idea of sorting the left- and right-hand tables into order and then scanning them in parallel. So, both data types must be capable of being fully ordered, and the join operator must be one that can only succeed for pairs of values that fall at the "same place" in the sort order. In practice this means that the join operator must behave like equality. But unlike hash join, where the left and right data types had better be the same (or at least bitwise equivalent), it is possible to merge-join two distinct data types so long as they are logically compatible. For example, the smallint-versus-integer equality operator is merge-joinable. We only need sorting operators that will bring both data types into a logically compatible sequence.
Execution of a merge join requires that the system be able to identify four operators related to the merge-join equality operator: less-than comparison for the left operand data type, less-than comparison for the right operand data type, less-than comparison between the two data types, and greater-than comparison between the two data types. (These are actually four distinct operators if the merge-joinable operator has two different operand data types; but when the operand types are the same the three less-than operators are all the same operator.) It is possible to specify these operators individually by name, as the SORT1, SORT2, LTCMP, and GTCMP options respectively. The system will fill in the default names <, <, <, > respectively if any of these are omitted when MERGES is specified. Also, MERGES will be assumed to be implied if any of these four operator options appear, so it is possible to specify just some of them and let the system fill in the rest.
The operand data types of the four comparison operators can be deduced from the operand types of the merge-joinable operator, so just as with COMMUTATOR, only the operator names need be given in these clauses. Unless you are using peculiar choices of operator names, it's sufficient to write MERGES and let the system fill in the details. (As with COMMUTATOR and NEGATOR, the system is able to make dummy operator entries if you happen to define the equality operator before the other ones.)
There are additional restrictions on operators that you mark merge-joinable. These restrictions are not currently checked by CREATE OPERATOR, but errors may occur when the operator is used if any are not true:
A merge-joinable equality operator must have a merge-joinable commutator (itself if the two operand data types are the same, or a related equality operator if they are different).
If there is a merge-joinable operator relating any two data types A and B, and another merge-joinable operator relating B to any third data type C, then A and C must also have a merge-joinable operator; in other words, having a merge-joinable operator must be transitive.
Bizarre results will ensue at runtime if the four comparison operators you name do not sort the data values compatibly.
Note: The function underlying a merge-joinable operator must be marked immutable or stable. If it is volatile, the system will never attempt to use the operator for a merge join.
Note: In PostgreSQL versions before 7.3, the MERGES shorthand was not available: to make a merge-joinable operator one had to write both SORT1 and SORT2 explicitly. Also, the LTCMP and GTCMP options did not exist; the names of those operators were hardwired as < and > respectively.