155 lines
11 KiB
HTML
155 lines
11 KiB
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<meta name="DC.Title" content="Summary of function references" />
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<meta name="abstract" content="For both qualified and unqualified function references, the function selection algorithm looks at all the applicable functions, both built-in and user-defined functions, that have the given name, the same number of defined parameters as argument, and each parameter identical to or promotable from the type of the corresponding argument." />
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<meta name="description" content="For both qualified and unqualified function references, the function selection algorithm looks at all the applicable functions, both built-in and user-defined functions, that have the given name, the same number of defined parameters as argument, and each parameter identical to or promotable from the type of the corresponding argument." />
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<meta name="DC.subject" content="UDFs (User-defined functions), summary of function references, function, references, summary for UDFs, general considerations, infix notation, infix notation and UDFs, LOB types, casting, casting, UDFs" />
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<meta name="keywords" content="UDFs (User-defined functions), summary of function references, function, references, summary for UDFs, general considerations, infix notation, infix notation and UDFs, LOB types, casting, casting, UDFs" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<title>Summary of function references</title>
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</head>
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<body id="rbafysummary"><a name="rbafysummary"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Summary of function references</h1>
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<div><p>For both qualified and unqualified function references, the function
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selection algorithm looks at all the applicable functions, both built-in and
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user-defined functions, that have the given name, the same number
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of defined parameters as argument, and each parameter identical to or promotable
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from the type of the corresponding argument.</p>
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<div class="section"><p>Applicable functions means functions in the named schema for a
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qualified reference, or functions in the schemas of the function path for
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an unqualified reference. The algorithm looks for an exact match, or failing
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that, a best match among these functions. The current function path is used,
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in the case of an unqualified reference only, as the deciding factor if two
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identically good matches are found in different schemas.</p>
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</div>
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<div class="section"><p>An interesting feature is the fact that function references can
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be nested, even references to the same function. This is generally true for
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built-in functions as well as UDFs. However, there are some limitations when
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column functions are involved. </p>
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</div>
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<div class="section"><p>Refining an earlier example:</p>
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<pre> <strong>CREATE FUNCTION</strong> BLOOP (<strong>INTEGER</strong>) <strong>RETURNS INTEGER</strong> ...
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<strong>CREATE FUNCTION</strong> BLOOP (<strong>DOUBLE</strong>) <strong>RETURNS INTEGER</strong> ...</pre>
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</div>
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<div class="section"><p>Now consider the following statement:</p>
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<pre> <strong>SELECT</strong> BLOOP( BLOOP(COLUMN1)) <strong>FROM</strong> T</pre>
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</div>
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<div class="section"><p>If COLUMN1 is a DECIMAL or DOUBLE column, the inner BLOOP reference
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resolves to the second BLOOP defined above. Because this BLOOP returns an
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INTEGER, the outer BLOOP resolves to the first BLOOP. </p>
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</div>
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<div class="section"><p>Alternatively, if COLUMN1 is a SMALLINT or INTEGER column, the
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inner BLOOP reference resolves to the first BLOOP defined above. Because
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this BLOOP returns an INTEGER, the outer BLOOP also resolves to the first
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BLOOP. In this case, you are seeing nested references to the same function. </p>
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</div>
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<div class="section"><div class="p">A few additional points important for function references are:
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<ul><li>You can define a function with the name of one of the SQL operators. For
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example, suppose you can attach some meaning to the <samp class="codeph">"+"</samp>
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operator for values which have distinct type BOAT. You can define the following
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UDF: <pre> <strong>CREATE FUNCTION</strong> "+" (BOAT, BOAT) <strong>RETURNS</strong> ...</pre>
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<div class="p">Then
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you can write the following valid SQL statement: <pre> <strong>SELECT</strong> "+"(BOAT_COL1, BOAT_COL2)
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<strong>FROM</strong> BIG_BOATS
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<strong>WHERE</strong> BOAT_OWNER = 'Nelson Mattos' </pre>
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</div>
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<p>You are
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not permitted to overload the built-in conditional operators such as <samp class="codeph">></samp>, <samp class="codeph">=</samp>,
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<samp class="codeph">LIKE</samp>, <samp class="codeph">IN</samp>, and so on, in this way.</p>
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</li>
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<li>The function selection algorithm does not consider the context of the
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reference in resolving to a particular function. Look at these BLOOP functions,
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modified a bit from before: <pre> <strong>CREATE FUNCTION</strong> BLOOP (<strong>INTEGER</strong>) <strong>RETURNS INTEGER</strong> ...
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<strong>CREATE FUNCTION</strong> BLOOP (<strong>DOUBLE</strong>) <strong>RETURNS CHAR</strong>(10)...</pre>
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<div class="p">Now suppose you write the following SELECT statement: <pre> <strong>SELECT</strong> 'ABCDEFG' <strong>CONCAT</strong> BLOOP(SMALLINT_COL) <strong>FROM</strong> T</pre>
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</div>
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<p>Because the best match, resolved using the SMALLINT argument, is
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the first BLOOP defined above, the second operand of the CONCAT resolves to
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data type INTEGER. The statement might not return the expected result since
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the returned integer will be cast as a VARCHAR before the CONCAT is performed.
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If the first BLOOP was not present, the other BLOOP is chosen and the statement
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execution is successful. </p>
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</li>
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<li>UDFs can be defined with parameters or results having any of the LOB types:
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BLOB, CLOB, or DBCLOB. The system will materialize the entire LOB value in
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storage before calling such a function, even if the source of the value is
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a <em>LOB locator</em> host variable. For example, consider the following
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fragment of a C language application: <pre> EXEC SQL <strong>BEGIN DECLARE SECTION</strong>;
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<strong>SQL TYPE IS CLOB</strong>(150K) clob150K ; /* LOB host var */
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<strong>SQL TYPE IS CLOB_LOCATOR</strong> clob_locator1; /* LOB locator host var */
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char string[40]; /* string host var */
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EXEC SQL <strong>END DECLARE SECTION</strong>; </pre>
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<div class="p">Either host variable <samp class="codeph">:clob150K</samp> or <samp class="codeph">:clob_locator1</samp>
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is valid as an argument for a function whose corresponding parameter is defined
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as <samp class="codeph">CLOB(500K)</samp>. Referring to the FINDSTRING defined in <a href="rbafyudfcfx2.htm#rbafyudfcfx2">Example: String search</a> both of the following are valid
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in the program: <pre> ... <strong>SELECT FINDSTRING</strong> (:clob150K, :string) <strong>FROM</strong> ...
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... <strong>SELECT FINDSTRING</strong> (:clob_locator1, :string) <strong>FROM</strong> ...</pre>
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</div>
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</li>
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<li>External UDF parameters or results which have one of the LOB types can
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be created with the AS LOCATOR modifier. In this case, the entire LOB value
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is not materialized before invocation. Instead, a LOB LOCATOR is passed to
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the UDF. <p>You can also use this capability on UDF parameters or results
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which have a distinct type that is based on a LOB. This capability is limited
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to external UDFs. Note that the argument to such a function can be any LOB
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value of the defined type; it does not need to be a host variable defined
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as one of the LOCATOR types. The use of host variable locators as arguments
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is completely unrelated to the use of AS LOCATOR in UDF parameters and result
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definitions.</p>
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</li>
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<li>UDFs can be defined with distinct types as parameters or as the result. DB2<sup>®</sup> will
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pass the value to the UDF in the format of the source data type of the distinct
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type. <div class="p">Distinct type values that originate in a host variable and which
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are used as arguments to a UDF which has its corresponding parameter defined
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as a distinct type must be explicitly cast to the distinct type by the user.
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There is no host language type for distinct types. DB2's strong typing necessitates
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this. Otherwise your results may be ambiguous. So, consider the BOAT distinct
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type that is defined over a BLOB that takes an object of type BOAT as its
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argument. In the following fragment of a C language application, the host
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variable <samp class="codeph">:ship</samp> holds the BLOB value that is to passed to
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the BOAT_COST function: <pre> EXEC SQL <strong>BEGIN DECLARE SECTION</strong>;
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<strong>SQL TYPE IS BLOB</strong>(150K) ship;
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EXEC SQL <strong>END DECLARE SECTION</strong>;</pre>
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</div>
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<div class="p">Both of the following
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statements correctly resolve to the BOAT_COST function, because both cast
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the <samp class="codeph">:ship</samp> host variable to type BOAT: <pre> ... <strong>SELECT</strong> BOAT_COST (<strong>BOAT</strong>(:ship)) <strong>FROM</strong> ...
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... <strong>SELECT</strong> BOAT_COST (<strong>CAST</strong>(:ship AS BOAT)) <strong>FROM</strong> ...</pre>
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</div>
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<p>If
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there are multiple BOAT distinct types in the database, or BOAT UDFs in other
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schema, you must be careful with your function path. Otherwise your results
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may be unpredictable. </p>
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</li>
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</ul>
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</div>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafyudfuudf.htm" title="Scalar and column UDFs can be called within an SQL statement almost everywhere that an expression is valid. Table UDFs can be called in the FROM clause of a SELECT. There are a few restrictions of UDF usage, however.">Use UDFs in SQL statements</a></div>
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</div>
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</div>
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</body>
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</html>
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