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<!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>44.5. Database Access from PL/Tcl</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="pltcl-global.html" title="44.4. Global Data in PL/Tcl" /><link rel="next" href="pltcl-trigger.html" title="44.6. Trigger Functions in PL/Tcl" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">44.5. Database Access from PL/Tcl</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pltcl-global.html" title="44.4. Global Data in PL/Tcl">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="pltcl.html" title="Chapter 44. PL/Tcl — Tcl Procedural Language">Up</a></td><th width="60%" align="center">Chapter 44. PL/Tcl — Tcl Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pltcl-trigger.html" title="44.6. Trigger Functions in PL/Tcl">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLTCL-DBACCESS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">44.5. Database Access from PL/Tcl <a href="#PLTCL-DBACCESS" class="id_link">#</a></h2></div></div></div><p>
In this section, we follow the usual Tcl convention of using question
marks, rather than brackets, to indicate an optional element in a
syntax synopsis. The following commands are available to access
the database from the body of a PL/Tcl function:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal"><code class="function">spi_exec</code> ?<span class="optional">-count <em class="replaceable"><code>n</code></em></span>? ?<span class="optional">-array <em class="replaceable"><code>name</code></em></span>? <em class="replaceable"><code>command</code></em> ?<span class="optional"><em class="replaceable"><code>loop-body</code></em></span>?</code></span></dt><dd><p>
Executes an SQL command given as a string. An error in the command
causes an error to be raised. Otherwise, the return value of <code class="function">spi_exec</code>
is the number of rows processed (selected, inserted, updated, or
deleted) by the command, or zero if the command is a utility
statement. In addition, if the command is a <code class="command">SELECT</code> statement, the
values of the selected columns are placed in Tcl variables as
described below.
</p><p>
The optional <code class="literal">-count</code> value tells
<code class="function">spi_exec</code> to stop
once <em class="replaceable"><code>n</code></em> rows have been retrieved,
much as if the query included a <code class="literal">LIMIT</code> clause.
If <em class="replaceable"><code>n</code></em> is zero, the query is run to
completion, the same as when <code class="literal">-count</code> is omitted.
</p><p>
If the command is a <code class="command">SELECT</code> statement, the values of the
result columns are placed into Tcl variables named after the columns.
If the <code class="literal">-array</code> option is given, the column values are
instead stored into elements of the named associative array, with the
column names used as array indexes. In addition, the current row
number within the result (counting from zero) is stored into the array
element named <span class="quote">“<span class="quote"><code class="literal">.tupno</code></span>”</span>, unless that name is
in use as a column name in the result.
</p><p>
If the command is a <code class="command">SELECT</code> statement and no <em class="replaceable"><code>loop-body</code></em>
script is given, then only the first row of results are stored into
Tcl variables or array elements; remaining rows, if any, are ignored.
No storing occurs if the query returns no rows. (This case can be
detected by checking the result of <code class="function">spi_exec</code>.)
For example:
</p><pre class="programlisting">
spi_exec "SELECT count(*) AS cnt FROM pg_proc"
</pre><p>
will set the Tcl variable <code class="literal">$cnt</code> to the number of rows in
the <code class="structname">pg_proc</code> system catalog.
</p><p>
If the optional <em class="replaceable"><code>loop-body</code></em> argument is given, it is
a piece of Tcl script that is executed once for each row in the
query result. (<em class="replaceable"><code>loop-body</code></em> is ignored if the given
command is not a <code class="command">SELECT</code>.)
The values of the current row's columns
are stored into Tcl variables or array elements before each iteration.
For example:
</p><pre class="programlisting">
spi_exec -array C "SELECT * FROM pg_class" {
elog DEBUG "have table $C(relname)"
}
</pre><p>
will print a log message for every row of <code class="literal">pg_class</code>. This
feature works similarly to other Tcl looping constructs; in
particular <code class="literal">continue</code> and <code class="literal">break</code> work in the
usual way inside the loop body.
</p><p>
If a column of a query result is null, the target
variable for it is <span class="quote">“<span class="quote">unset</span>”</span> rather than being set.
</p></dd><dt><span class="term"><code class="function">spi_prepare</code> <em class="replaceable"><code>query</code></em> <em class="replaceable"><code>typelist</code></em></span></dt><dd><p>
Prepares and saves a query plan for later execution. The
saved plan will be retained for the life of the current
session.<a id="id-1.8.9.9.2.1.2.2.1.1" class="indexterm"></a>
</p><p>
The query can use parameters, that is, placeholders for
values to be supplied whenever the plan is actually executed.
In the query string, refer to parameters
by the symbols <code class="literal">$1</code> ... <code class="literal">$<em class="replaceable"><code>n</code></em></code>.
If the query uses parameters, the names of the parameter types
must be given as a Tcl list. (Write an empty list for
<em class="replaceable"><code>typelist</code></em> if no parameters are used.)
</p><p>
The return value from <code class="function">spi_prepare</code> is a query ID
to be used in subsequent calls to <code class="function">spi_execp</code>. See
<code class="function">spi_execp</code> for an example.
</p></dd><dt><span class="term"><code class="literal"><code class="function">spi_execp</code> ?<span class="optional">-count <em class="replaceable"><code>n</code></em></span>? ?<span class="optional">-array <em class="replaceable"><code>name</code></em></span>? ?<span class="optional">-nulls <em class="replaceable"><code>string</code></em></span>? <em class="replaceable"><code>queryid</code></em> ?<span class="optional"><em class="replaceable"><code>value-list</code></em></span>? ?<span class="optional"><em class="replaceable"><code>loop-body</code></em></span>?</code></span></dt><dd><p>
Executes a query previously prepared with <code class="function">spi_prepare</code>.
<em class="replaceable"><code>queryid</code></em> is the ID returned by
<code class="function">spi_prepare</code>. If the query references parameters,
a <em class="replaceable"><code>value-list</code></em> must be supplied. This
is a Tcl list of actual values for the parameters. The list must be
the same length as the parameter type list previously given to
<code class="function">spi_prepare</code>. Omit <em class="replaceable"><code>value-list</code></em>
if the query has no parameters.
</p><p>
The optional value for <code class="literal">-nulls</code> is a string of spaces and
<code class="literal">'n'</code> characters telling <code class="function">spi_execp</code>
which of the parameters are null values. If given, it must have exactly the
same length as the <em class="replaceable"><code>value-list</code></em>. If it
is not given, all the parameter values are nonnull.
</p><p>
Except for the way in which the query and its parameters are specified,
<code class="function">spi_execp</code> works just like <code class="function">spi_exec</code>.
The <code class="literal">-count</code>, <code class="literal">-array</code>, and
<em class="replaceable"><code>loop-body</code></em> options are the same,
and so is the result value.
</p><p>
Here's an example of a PL/Tcl function using a prepared plan:
</p><pre class="programlisting">
CREATE FUNCTION t1_count(integer, integer) RETURNS integer AS $$
if {![ info exists GD(plan) ]} {
# prepare the saved plan on the first call
set GD(plan) [ spi_prepare \
"SELECT count(*) AS cnt FROM t1 WHERE num >= \$1 AND num <= \$2" \
[ list int4 int4 ] ]
}
spi_execp -count 1 $GD(plan) [ list $1 $2 ]
return $cnt
$$ LANGUAGE pltcl;
</pre><p>
We need backslashes inside the query string given to
<code class="function">spi_prepare</code> to ensure that the
<code class="literal">$<em class="replaceable"><code>n</code></em></code> markers will be passed
through to <code class="function">spi_prepare</code> as-is, and not replaced by Tcl
variable substitution.
</p></dd><dt><span class="term"><code class="function">subtransaction</code> <em class="replaceable"><code>command</code></em></span></dt><dd><p>
The Tcl script contained in <em class="replaceable"><code>command</code></em> is
executed within an SQL subtransaction. If the script returns an
error, that entire subtransaction is rolled back before returning the
error out to the surrounding Tcl code.
See <a class="xref" href="pltcl-subtransactions.html" title="44.9. Explicit Subtransactions in PL/Tcl">Section 44.9</a> for more details and an
example.
</p></dd><dt><span class="term"><code class="function">quote</code> <em class="replaceable"><code>string</code></em></span></dt><dd><p>
Doubles all occurrences of single quote and backslash characters
in the given string. This can be used to safely quote strings
that are to be inserted into SQL commands given
to <code class="function">spi_exec</code> or
<code class="function">spi_prepare</code>.
For example, think about an SQL command string like:
</p><pre class="programlisting">
"SELECT '$val' AS ret"
</pre><p>
where the Tcl variable <code class="literal">val</code> actually contains
<code class="literal">doesn't</code>. This would result
in the final command string:
</p><pre class="programlisting">
SELECT 'doesn't' AS ret
</pre><p>
which would cause a parse error during
<code class="function">spi_exec</code> or
<code class="function">spi_prepare</code>.
To work properly, the submitted command should contain:
</p><pre class="programlisting">
SELECT 'doesn''t' AS ret
</pre><p>
which can be formed in PL/Tcl using:
</p><pre class="programlisting">
"SELECT '[ quote $val ]' AS ret"
</pre><p>
One advantage of <code class="function">spi_execp</code> is that you don't
have to quote parameter values like this, since the parameters are never
parsed as part of an SQL command string.
</p></dd><dt><span class="term">
<code class="function">elog</code> <em class="replaceable"><code>level</code></em> <em class="replaceable"><code>msg</code></em>
<a id="id-1.8.9.9.2.1.6.1.4" class="indexterm"></a>
</span></dt><dd><p>
Emits a log or error message. Possible levels are
<code class="literal">DEBUG</code>, <code class="literal">LOG</code>, <code class="literal">INFO</code>,
<code class="literal">NOTICE</code>, <code class="literal">WARNING</code>, <code class="literal">ERROR</code>, and
<code class="literal">FATAL</code>. <code class="literal">ERROR</code>
raises an error condition; if this is not trapped by the surrounding
Tcl code, the error propagates out to the calling query, causing
the current transaction or subtransaction to be aborted. This
is effectively the same as the Tcl <code class="literal">error</code> command.
<code class="literal">FATAL</code> aborts the transaction and causes the current
session to shut down. (There is probably no good reason to use
this error level in PL/Tcl functions, but it's provided for
completeness.) The other levels only generate messages of different
priority levels.
Whether messages of a particular priority are reported to the client,
written to the server log, or both is controlled by the
<a class="xref" href="runtime-config-logging.html#GUC-LOG-MIN-MESSAGES">log_min_messages</a> and
<a class="xref" href="runtime-config-client.html#GUC-CLIENT-MIN-MESSAGES">client_min_messages</a> configuration
variables. See <a class="xref" href="runtime-config.html" title="Chapter 20. Server Configuration">Chapter 20</a>
and <a class="xref" href="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl">Section 44.8</a>
for more information.
</p></dd></dl></div><p>
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