|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
RRP £32.00 ($49.95)
Sales of this book support the PostgreSQL project! Get a printed copy>>>
The specific operator to be used in an operator invocation is determined by following the procedure below. Note that this procedure is indirectly affected by the precedence of the involved operators. See section 2.1.6 Lexical Precedence for more information.
Operator Type Resolution:
Select the operators to be considered from the
pg_operatorsystem catalog. If an unqualified operator name was used (the usual case), the operators considered are those of the right name and argument count that are visible in the current search path (see section 3.7.3 The Schema Search Path). If a qualified operator name was given, only operators in the specified schema are considered.
- If the search path finds multiple operators of identical argument types, only the one appearing earliest in the path is considered. But operators of different argument types are considered on an equal footing regardless of search path position.
Check for an operator accepting exactly the input argument types.
If one exists (there can be only one exact match in the set of
operators considered), use it.
If one argument of a binary operator invocation is of the
unknowntype, then assume it is the same type as the other argument for this check. Other cases involving
unknownwill never find a match at this step.
- If one argument of a binary operator invocation is of the
Look for the best match.
Discard candidate operators for which the input types do not match
and cannot be converted (using an implicit conversion) to match.
unknownliterals are assumed to be convertible to anything for this purpose. If only one candidate remains, use it; else continue to the next step.
- Run through all candidates and keep those with the most exact matches on input types. (Domains are considered the same as their base type for this purpose.) Keep all candidates if none have any exact matches. If only one candidate remains, use it; else continue to the next step.
- Run through all candidates and keep those that accept preferred types (of the input data type's type category) at the most positions where type conversion will be required. Keep all candidates if none accept preferred types. If only one candidate remains, use it; else continue to the next step.
If any input arguments are
unknown, check the type categories accepted at those argument positions by the remaining candidates. At each position, select the
stringcategory if any candidate accepts that category. (This bias towards string is appropriate since an unknown-type literal does look like a string.) Otherwise, if all the remaining candidates accept the same type category, select that category; otherwise fail because the correct choice cannot be deduced without more clues. Now discard candidates that do not accept the selected type category. Furthermore, if any candidate accepts a preferred type at a given argument position, discard candidates that accept non-preferred types for that argument.
- If only one candidate remains, use it. If no candidate or more than one candidate remains, then fail.
- Discard candidate operators for which the input types do not match and cannot be converted (using an implicit conversion) to match.
Some examples follow.
Exponentiation Operator Type Resolution:
There is only one exponentiation
operator defined in the catalog, and it takes arguments of type
The scanner assigns an initial type of
integer to both arguments
of this query expression:
SELECT 2 ^ 3 AS "exp"; exp ----- 8 (1 row)
So the parser does a type conversion on both operands and the query is equivalent to
SELECT CAST(2 AS double precision) ^ CAST(3 AS double precision) AS "exp";
String Concatenation Operator Type Resolution:
A string-like syntax is used for working with string types as well as for working with complex extension types. Strings with unspecified type are matched with likely operator candidates.
An example with one unspecified argument:
SELECT text 'abc' || 'def' AS "text and unknown"; text and unknown ------------------ abcdef (1 row)
In this case the parser looks to see if there is an operator taking
for both arguments. Since there is, it assumes that the second argument should
be interpreted as of type
Here is a concatenation on unspecified types:
SELECT 'abc' || 'def' AS "unspecified"; unspecified ------------- abcdef (1 row)
In this case there is no initial hint for which type to use, since no types
are specified in the query. So, the parser looks for all candidate operators
and finds that there are candidates accepting both string-category and
bit-string-category inputs. Since string category is preferred when available,
that category is selected, and then the
preferred type for strings,
text, is used as the specific
type to resolve the unknown literals to.
Absolute-Value and Negation Operator Type Resolution:
The PostgreSQL operator catalog has several
entries for the prefix operator
@, all of which implement
absolute-value operations for various numeric data types. One of these
entries is for type
float8, which is the preferred type in
the numeric category. Therefore, PostgreSQL
will use that entry when faced with a non-numeric input:
SELECT @ '-4.5' AS "abs"; abs ----- 4.5 (1 row)
Here the system has performed an implicit conversion from
before applying the chosen operator. We can verify that
not some other type was used:
SELECT @ '-4.5e500' AS "abs"; ERROR: "-4.5e500" is out of range for type double precision
On the other hand, the prefix operator
~ (bitwise negation)
is defined only for integer data types, not for
float8. So, if we
try a similar case with
~, we get:
SELECT ~ '20' AS "negation"; ERROR: operator is not unique: ~ "unknown" HINT: Could not choose a best candidate operator. You may need to add explicit type casts.
This happens because the system can't decide which of the several
~ operators should be preferred. We can help
it out with an explicit cast:
SELECT ~ CAST('20' AS int8) AS "negation"; negation ---------- -21 (1 row)
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|