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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
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<div><div><h2 class="title" style="clear: both">
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<a name="pltcl-dbaccess"></a>36.5.�Database Access from PL/Tcl</h2></div></div>
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<p> The following commands are available to access the database from
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the body of a PL/Tcl function:
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<div class="variablelist"><dl>
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<dt><span class="term"><tt class="literal"><tt class="function">spi_exec</tt>
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?<span class="optional">-count <i class="replaceable"><tt>n</tt></i></span>?
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?<span class="optional">-array <i class="replaceable"><tt>name</tt></i></span>?
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<i class="replaceable"><tt>command</tt></i>
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?<span class="optional"><i class="replaceable"><tt>loop-body</tt></i></span>?
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<p> Executes an SQL command given as a string. An error in the command
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causes an error to be raised. Otherwise, the return value of <tt class="function">spi_exec</tt>
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is the number of rows processed (selected, inserted, updated, or
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deleted) by the command, or zero if the command is a utility
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statement. In addition, if the command is a <tt class="command">SELECT</tt> statement, the
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values of the selected columns are placed in Tcl variables as
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<p> The optional <tt class="literal">-count</tt> value tells
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<tt class="function">spi_exec</tt> the maximum number of rows
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to process in the command. The effect of this is comparable to
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setting up a query as a cursor and then saying <tt class="literal">FETCH <i class="replaceable"><tt>n</tt></i></tt>.
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<p> If the command is a <tt class="command">SELECT</tt> statement, the values of the
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result columns are placed into Tcl variables named after the columns.
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If the <tt class="literal">-array</tt> option is given, the column values are
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instead stored into the named associative array, with the
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column names used as array indexes.
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<p> If the command is a <tt class="command">SELECT</tt> statement and no <i class="replaceable"><tt>loop-body</tt></i>
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script is given, then only the first row of results are stored into
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Tcl variables; remaining rows, if any, are ignored. No storing occurs
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query returns no rows. (This case can be detected by checking the
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result of <tt class="function">spi_exec</tt>.) For example,
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<pre class="programlisting">spi_exec "SELECT count(*) AS cnt FROM pg_proc"</pre>
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will set the Tcl variable <tt class="literal">$cnt</tt> to the number of rows in
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the <tt class="structname">pg_proc</tt> system catalog.
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<p> If the optional <i class="replaceable"><tt>loop-body</tt></i> argument is given, it is
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a piece of Tcl script that is executed once for each row in the
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query result. (<i class="replaceable"><tt>loop-body</tt></i> is ignored if the given
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command is not a <tt class="command">SELECT</tt>.) The values of the current row's columns
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are stored into Tcl variables before each iteration. For example,
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<pre class="programlisting">spi_exec -array C "SELECT * FROM pg_class" {
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elog DEBUG "have table $C(relname)"
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will print a log message for every row of <tt class="literal">pg_class</tt>. This
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feature works similarly to other Tcl looping constructs; in
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particular <tt class="literal">continue</tt> and <tt class="literal">break</tt> work in the
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usual way inside the loop body.
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<p> If a column of a query result is null, the target
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variable for it is “<span class="quote">unset</span>” rather than being set.
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<dt><span class="term"><tt class="function">spi_prepare</tt> <i class="replaceable"><tt>query</tt></i> <i class="replaceable"><tt>typelist</tt></i></span></dt>
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<p> Prepares and saves a query plan for later execution. The
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saved plan will be retained for the life of the current
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session.<a name="id2651635"></a>
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<p> The query may use parameters, that is, placeholders for
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values to be supplied whenever the plan is actually executed.
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In the query string, refer to parameters
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by the symbols <tt class="literal">$1</tt> ... <tt class="literal">$<i class="replaceable"><tt>n</tt></i></tt>.
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If the query uses parameters, the names of the parameter types
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must be given as a Tcl list. (Write an empty list for
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<i class="replaceable"><tt>typelist</tt></i> if no parameters are used.)
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Presently, the parameter types must be identified by the internal
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type names shown in the system table <tt class="literal">pg_type</tt>; for example <tt class="literal">int4</tt> not
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<tt class="literal">integer</tt>.
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<p> The return value from <tt class="function">spi_prepare</tt> is a query ID
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to be used in subsequent calls to <tt class="function">spi_execp</tt>. See
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<tt class="function">spi_execp</tt> for an example.
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<dt><span class="term"><tt class="literal"><tt class="function">spi_execp</tt>
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?<span class="optional">-count <i class="replaceable"><tt>n</tt></i></span>?
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?<span class="optional">-array <i class="replaceable"><tt>name</tt></i></span>?
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?<span class="optional">-nulls <i class="replaceable"><tt>string</tt></i></span>?
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<i class="replaceable"><tt>queryid</tt></i>
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?<span class="optional"><i class="replaceable"><tt>value-list</tt></i></span>?
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?<span class="optional"><i class="replaceable"><tt>loop-body</tt></i></span>?
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<p> Executes a query previously prepared with <tt class="function">spi_prepare</tt>.
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<i class="replaceable"><tt>queryid</tt></i> is the ID returned by
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<tt class="function">spi_prepare</tt>. If the query references parameters,
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a <i class="replaceable"><tt>value-list</tt></i> must be supplied. This
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is a Tcl list of actual values for the parameters. The list must be
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the same length as the parameter type list previously given to
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<tt class="function">spi_prepare</tt>. Omit <i class="replaceable"><tt>value-list</tt></i>
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if the query has no parameters.
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<p> The optional value for <tt class="literal">-nulls</tt> is a string of spaces and
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<tt class="literal">'n'</tt> characters telling <tt class="function">spi_execp</tt>
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which of the parameters are null values. If given, it must have exactly the
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same length as the <i class="replaceable"><tt>value-list</tt></i>. If it
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is not given, all the parameter values are nonnull.
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<p> Except for the way in which the query and its parameters are specified,
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<tt class="function">spi_execp</tt> works just like <tt class="function">spi_exec</tt>.
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The <tt class="literal">-count</tt>, <tt class="literal">-array</tt>, and
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<i class="replaceable"><tt>loop-body</tt></i> options are the same,
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and so is the result value.
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<p> Here's an example of a PL/Tcl function using a prepared plan:
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<pre class="programlisting">CREATE FUNCTION t1_count(integer, integer) RETURNS integer AS $$
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if {![ info exists GD(plan) ]} {
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# prepare the saved plan on the first call
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set GD(plan) [ spi_prepare \
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"SELECT count(*) AS cnt FROM t1 WHERE num >= \$1 AND num <= \$2" \
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spi_execp -count 1 $GD(plan) [ list $1 $2 ]
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$$ LANGUAGE pltcl;</pre>
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We need backslashes inside the query string given to
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<tt class="function">spi_prepare</tt> to ensure that the
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<tt class="literal">$<i class="replaceable"><tt>n</tt></i></tt> markers will be passed
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through to <tt class="function">spi_prepare</tt> as-is, and not replaced by Tcl
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variable substitution.
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<dt><span class="term"><tt class="function">spi_lastoid</tt></span></dt>
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<dd><p> Returns the OID of the row inserted by the last
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<tt class="function">spi_exec</tt> or <tt class="function">spi_execp</tt>,
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if the command was a single-row <tt class="command">INSERT</tt>. (If not, you get zero.)
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<dt><span class="term"><tt class="function">quote</tt> <i class="replaceable"><tt>string</tt></i></span></dt>
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<p> Doubles all occurrences of single quote and backslash characters
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in the given string. This may be used to safely quote strings
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that are to be inserted into SQL commands given
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to <tt class="function">spi_exec</tt> or
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<tt class="function">spi_prepare</tt>.
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For example, think about an SQL command string like
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<pre class="programlisting">"SELECT '$val' AS ret"</pre>
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where the Tcl variable <tt class="literal">val</tt> actually contains
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<tt class="literal">doesn't</tt>. This would result
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in the final command string
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<pre class="programlisting">SELECT 'doesn't' AS ret</pre>
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which would cause a parse error during
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<tt class="function">spi_exec</tt> or
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<tt class="function">spi_prepare</tt>.
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To work properly, the submitted command should contain
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<pre class="programlisting">SELECT 'doesn''t' AS ret</pre>
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which can be formed in PL/Tcl using
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<pre class="programlisting">"SELECT '[ quote $val ]' AS ret"</pre>
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One advantage of <tt class="function">spi_execp</tt> is that you don't
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have to quote parameter values like this, since the parameters are never
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parsed as part of an SQL command string.
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<dt><span class="term"><tt class="function">elog</tt> <i class="replaceable"><tt>level</tt></i> <i class="replaceable"><tt>msg</tt></i></span></dt>
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<dd><p> Emits a log or error message. Possible levels are
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<tt class="literal">DEBUG</tt>, <tt class="literal">LOG</tt>, <tt class="literal">INFO</tt>,
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<tt class="literal">NOTICE</tt>, <tt class="literal">WARNING</tt>, <tt class="literal">ERROR</tt>, and
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<tt class="literal">FATAL</tt>. Most simply emit the given message just like
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the <tt class="literal">elog</tt> C function. <tt class="literal">ERROR</tt>
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raises an error condition; if this is not trapped by the surrounding
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Tcl code, the error propagates out to the calling query, causing
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the current transaction or subtransaction to be aborted. This
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is effectively the same as the Tcl <tt class="literal">error</tt> command.
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<tt class="literal">FATAL</tt> aborts the transaction and causes the current
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session to shut down. (There is probably no good reason to use
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this error level in PL/Tcl functions, but it's provided for
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docs/en_US/pg/pltcl-dbaccess.html
