|The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 454 pages
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The specific operator that is referenced by an operator expression is determined using the following procedure. Note that this procedure is indirectly affected by the precedence of the involved operators, since that will determine which sub-expressions are taken to be the inputs of which 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 a non-schema-qualified operator name was used (the usual case), the operators considered are those with the matching 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 with identical argument types, only the one appearing earliest in the path is considered. Operators with 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. Invocations involving two
unknowninputs, or a unary operator with an
unknowninput, will 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 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 looks 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 in that category, 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.
Factorial Operator Type Resolution:
There is only one factorial operator (postfix
defined in the standard catalog, and it takes an argument of type
The scanner assigns an initial type of
integer to the argument
in this query expression:
SELECT 40 ! AS "40 factorial"; 40 factorial -------------------------------------------------- 815915283247897734345611269596115894272000000000 (1 row)
So the parser does a type conversion on the operand and the query is equivalent to:
SELECT CAST(40 AS bigint) ! AS "40 factorial";
String Concatenation Operator Type Resolution:
A string-like syntax is used for working with string types and 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 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 as.
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 an
SELECT @ '-4.5' AS "abs"; abs ----- 4.5 (1 row)
Here the system has implicitly resolved the unknown-type literal as type
float8 before applying the chosen operator. We can verify that
float8 and 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 might need to add explicit type casts.
This happens because the system cannot 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)
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