diff options
Diffstat (limited to 'doc/src/sgml/html/plhandler.html')
| -rw-r--r-- | doc/src/sgml/html/plhandler.html | 156 |
1 files changed, 156 insertions, 0 deletions
diff --git a/doc/src/sgml/html/plhandler.html b/doc/src/sgml/html/plhandler.html new file mode 100644 index 0000000..2e80bef --- /dev/null +++ b/doc/src/sgml/html/plhandler.html @@ -0,0 +1,156 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 58. Writing a Procedural Language Handler</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="nls-programmer.html" title="57.2. For the Programmer" /><link rel="next" href="fdwhandler.html" title="Chapter 59. Writing a Foreign Data Wrapper" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 58. Writing a Procedural Language Handler</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="nls-programmer.html" title="57.2. For the Programmer">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><th width="60%" align="center">Part VII. Internals</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="fdwhandler.html" title="Chapter 59. Writing a Foreign Data Wrapper">Next</a></td></tr></table><hr /></div><div class="chapter" id="PLHANDLER"><div class="titlepage"><div><div><h2 class="title">Chapter 58. Writing a Procedural Language Handler</h2></div></div></div><a id="id-1.10.9.2" class="indexterm"></a><p> + All calls to functions that are written in a language other than + the current <span class="quote">“<span class="quote">version 1</span>”</span> interface for compiled + languages (this includes functions in user-defined procedural languages + and functions written in SQL) go through a <em class="firstterm">call handler</em> + function for the specific language. It is the responsibility of + the call handler to execute the function in a meaningful way, such + as by interpreting the supplied source text. This chapter outlines + how a new procedural language's call handler can be written. + </p><p> + The call handler for a procedural language is a + <span class="quote">“<span class="quote">normal</span>”</span> function that must be written in a compiled + language such as C, using the version-1 interface, and registered + with <span class="productname">PostgreSQL</span> as taking no arguments + and returning the type <code class="type">language_handler</code>. This + special pseudo-type identifies the function as a call handler and + prevents it from being called directly in SQL commands. + For more details on C language calling conventions and dynamic loading, + see <a class="xref" href="xfunc-c.html" title="38.10. C-Language Functions">Section 38.10</a>. + </p><p> + The call handler is called in the same way as any other function: + It receives a pointer to a + <code class="structname">FunctionCallInfoBaseData</code> <code class="type">struct</code> containing + argument values and information about the called function, and it + is expected to return a <code class="type">Datum</code> result (and possibly + set the <code class="structfield">isnull</code> field of the + <code class="structname">FunctionCallInfoBaseData</code> structure, if it wishes + to return an SQL null result). The difference between a call + handler and an ordinary callee function is that the + <code class="structfield">flinfo->fn_oid</code> field of the + <code class="structname">FunctionCallInfoBaseData</code> structure will contain + the OID of the actual function to be called, not of the call + handler itself. The call handler must use this field to determine + which function to execute. Also, the passed argument list has + been set up according to the declaration of the target function, + not of the call handler. + </p><p> + It's up to the call handler to fetch the entry of the function from the + <code class="classname">pg_proc</code> system catalog and to analyze the argument + and return types of the called function. The <code class="literal">AS</code> clause from the + <code class="command">CREATE FUNCTION</code> command for the function will be found + in the <code class="literal">prosrc</code> column of the + <code class="classname">pg_proc</code> row. This is commonly source + text in the procedural language, but in theory it could be something else, + such as a path name to a file, or anything else that tells the call handler + what to do in detail. + </p><p> + Often, the same function is called many times per SQL statement. + A call handler can avoid repeated lookups of information about the + called function by using the + <code class="structfield">flinfo->fn_extra</code> field. This will + initially be <code class="symbol">NULL</code>, but can be set by the call handler to point at + information about the called function. On subsequent calls, if + <code class="structfield">flinfo->fn_extra</code> is already non-<code class="symbol">NULL</code> + then it can be used and the information lookup step skipped. The + call handler must make sure that + <code class="structfield">flinfo->fn_extra</code> is made to point at + memory that will live at least until the end of the current query, + since an <code class="structname">FmgrInfo</code> data structure could be + kept that long. One way to do this is to allocate the extra data + in the memory context specified by + <code class="structfield">flinfo->fn_mcxt</code>; such data will + normally have the same lifespan as the + <code class="structname">FmgrInfo</code> itself. But the handler could + also choose to use a longer-lived memory context so that it can cache + function definition information across queries. + </p><p> + When a procedural-language function is invoked as a trigger, no arguments + are passed in the usual way, but the + <code class="structname">FunctionCallInfoBaseData</code>'s + <code class="structfield">context</code> field points at a + <code class="structname">TriggerData</code> structure, rather than being <code class="symbol">NULL</code> + as it is in a plain function call. A language handler should + provide mechanisms for procedural-language functions to get at the trigger + information. + </p><p> + A template for a procedural-language handler written as a C extension is + provided in <code class="literal">src/test/modules/plsample</code>. This is a + working sample demonstrating one way to create a procedural-language + handler, process parameters, and return a value. + </p><p> + Although providing a call handler is sufficient to create a minimal + procedural language, there are two other functions that can optionally + be provided to make the language more convenient to use. These + are a <em class="firstterm">validator</em> and an + <em class="firstterm">inline handler</em>. A validator can be provided + to allow language-specific checking to be done during + <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>. + An inline handler can be provided to allow the language to support + anonymous code blocks executed via the <a class="xref" href="sql-do.html" title="DO"><span class="refentrytitle">DO</span></a> command. + </p><p> + If a validator is provided by a procedural language, it + must be declared as a function taking a single parameter of type + <code class="type">oid</code>. The validator's result is ignored, so it is customarily + declared to return <code class="type">void</code>. The validator will be called at + the end of a <code class="command">CREATE FUNCTION</code> command that has created + or updated a function written in the procedural language. + The passed-in OID is the OID of the function's <code class="classname">pg_proc</code> + row. The validator must fetch this row in the usual way, and do + whatever checking is appropriate. + First, call <code class="function">CheckFunctionValidatorAccess()</code> to diagnose + explicit calls to the validator that the user could not achieve through + <code class="command">CREATE FUNCTION</code>. Typical checks then include verifying + that the function's argument and result types are supported by the + language, and that the function's body is syntactically correct + in the language. If the validator finds the function to be okay, + it should just return. If it finds an error, it should report that + via the normal <code class="function">ereport()</code> error reporting mechanism. + Throwing an error will force a transaction rollback and thus prevent + the incorrect function definition from being committed. + </p><p> + Validator functions should typically honor the <a class="xref" href="runtime-config-client.html#GUC-CHECK-FUNCTION-BODIES">check_function_bodies</a> parameter: if it is turned off then + any expensive or context-sensitive checking should be skipped. If the + language provides for code execution at compilation time, the validator + must suppress checks that would induce such execution. In particular, + this parameter is turned off by <span class="application">pg_dump</span> so that it can + load procedural language functions without worrying about side effects or + dependencies of the function bodies on other database objects. + (Because of this requirement, the call handler should avoid + assuming that the validator has fully checked the function. The point + of having a validator is not to let the call handler omit checks, but + to notify the user immediately if there are obvious errors in a + <code class="command">CREATE FUNCTION</code> command.) + While the choice of exactly what to check is mostly left to the + discretion of the validator function, note that the core + <code class="command">CREATE FUNCTION</code> code only executes <code class="literal">SET</code> clauses + attached to a function when <code class="varname">check_function_bodies</code> is on. + Therefore, checks whose results might be affected by GUC parameters + definitely should be skipped when <code class="varname">check_function_bodies</code> is + off, to avoid false failures when restoring a dump. + </p><p> + If an inline handler is provided by a procedural language, it + must be declared as a function taking a single parameter of type + <code class="type">internal</code>. The inline handler's result is ignored, so it is + customarily declared to return <code class="type">void</code>. The inline handler + will be called when a <code class="command">DO</code> statement is executed specifying + the procedural language. The parameter actually passed is a pointer + to an <code class="structname">InlineCodeBlock</code> struct, which contains information + about the <code class="command">DO</code> statement's parameters, in particular the + text of the anonymous code block to be executed. The inline handler + should execute this code and return. + </p><p> + It's recommended that you wrap all these function declarations, + as well as the <code class="command">CREATE LANGUAGE</code> command itself, into + an <em class="firstterm">extension</em> so that a simple <code class="command">CREATE EXTENSION</code> + command is sufficient to install the language. See + <a class="xref" href="extend-extensions.html" title="38.17. Packaging Related Objects into an Extension">Section 38.17</a> for information about writing + extensions. + </p><p> + The procedural languages included in the standard distribution + are good references when trying to write your own language handler. + Look into the <code class="filename">src/pl</code> subdirectory of the source tree. + The <a class="xref" href="sql-createlanguage.html" title="CREATE LANGUAGE"><span class="refentrytitle">CREATE LANGUAGE</span></a> + reference page also has some useful details. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="nls-programmer.html" title="57.2. For the Programmer">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="fdwhandler.html" title="Chapter 59. Writing a Foreign Data Wrapper">Next</a></td></tr><tr><td width="40%" align="left" valign="top">57.2. For the Programmer </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 59. Writing a Foreign Data Wrapper</td></tr></table></div></body></html>
\ No newline at end of file |