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><A
NAME="PLTCL-DBACCESS"
>40.5. Database Access from PL/Tcl</A
></H1
><P
> The following commands are available to access the database from
the body of a PL/Tcl function:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>spi_exec</CODE
> ?<SPAN
CLASS="OPTIONAL"
>-count <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></SPAN
>? ?<SPAN
CLASS="OPTIONAL"
>-array <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></SPAN
>? <TT
CLASS="REPLACEABLE"
><I
>command</I
></TT
> ?<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
></SPAN
>?</TT
></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 <TT
CLASS="COMMAND"
>SELECT</TT
> statement, the
values of the selected columns are placed in Tcl variables as
described below.
</P
><P
> The optional <TT
CLASS="LITERAL"
>-count</TT
> value tells
<CODE
CLASS="FUNCTION"
>spi_exec</CODE
> the maximum number of rows
to process in the command. The effect of this is comparable to
setting up a query as a cursor and then saying <TT
CLASS="LITERAL"
>FETCH <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
>.
</P
><P
> If the command is a <TT
CLASS="COMMAND"
>SELECT</TT
> statement, the values of the
result columns are placed into Tcl variables named after the columns.
If the <TT
CLASS="LITERAL"
>-array</TT
> 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"
>"<TT
CLASS="LITERAL"
>.tupno</TT
>"</SPAN
>, unless that name is
in use as a column name in the result.
</P
><P
> If the command is a <TT
CLASS="COMMAND"
>SELECT</TT
> statement and no <TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
>
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 <TT
CLASS="LITERAL"
>$cnt</TT
> to the number of rows in
the <TT
CLASS="STRUCTNAME"
>pg_proc</TT
> system catalog.
</P
><P
> If the optional <TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
> argument is given, it is
a piece of Tcl script that is executed once for each row in the
query result. (<TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
> is ignored if the given
command is not a <TT
CLASS="COMMAND"
>SELECT</TT
>.)
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 <TT
CLASS="LITERAL"
>pg_class</TT
>. This
feature works similarly to other Tcl looping constructs; in
particular <TT
CLASS="LITERAL"
>continue</TT
> and <TT
CLASS="LITERAL"
>break</TT
> 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"
>"unset"</SPAN
> rather than being set.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>spi_prepare</CODE
> <TT
CLASS="REPLACEABLE"
><I
>query</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>typelist</I
></TT
></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.
</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 <TT
CLASS="LITERAL"
>$1</TT
> ... <TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
>.
If the query uses parameters, the names of the parameter types
must be given as a Tcl list. (Write an empty list for
<TT
CLASS="REPLACEABLE"
><I
>typelist</I
></TT
> 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
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>spi_execp</CODE
> ?<SPAN
CLASS="OPTIONAL"
>-count <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></SPAN
>? ?<SPAN
CLASS="OPTIONAL"
>-array <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></SPAN
>? ?<SPAN
CLASS="OPTIONAL"
>-nulls <TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
></SPAN
>? <TT
CLASS="REPLACEABLE"
><I
>queryid</I
></TT
> ?<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>value-list</I
></TT
></SPAN
>? ?<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
></SPAN
>?</TT
></DT
><DD
><P
> Executes a query previously prepared with <CODE
CLASS="FUNCTION"
>spi_prepare</CODE
>.
<TT
CLASS="REPLACEABLE"
><I
>queryid</I
></TT
> is the ID returned by
<CODE
CLASS="FUNCTION"
>spi_prepare</CODE
>. If the query references parameters,
a <TT
CLASS="REPLACEABLE"
><I
>value-list</I
></TT
> 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 <TT
CLASS="REPLACEABLE"
><I
>value-list</I
></TT
>
if the query has no parameters.
</P
><P
> The optional value for <TT
CLASS="LITERAL"
>-nulls</TT
> is a string of spaces and
<TT
CLASS="LITERAL"
>'n'</TT
> 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 <TT
CLASS="REPLACEABLE"
><I
>value-list</I
></TT
>. 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 <TT
CLASS="LITERAL"
>-count</TT
>, <TT
CLASS="LITERAL"
>-array</TT
>, and
<TT
CLASS="REPLACEABLE"
><I
>loop-body</I
></TT
> 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
<TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
> markers will be passed
through to <CODE
CLASS="FUNCTION"
>spi_prepare</CODE
> as-is, and not replaced by Tcl
variable substitution.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>spi_lastoid</CODE
></DT
><DD
><P
> Returns the OID of the row inserted by the last
<CODE
CLASS="FUNCTION"
>spi_exec</CODE
> or <CODE
CLASS="FUNCTION"
>spi_execp</CODE
>, if the
command was a single-row <TT
CLASS="COMMAND"
>INSERT</TT
> and the modified
table contained OIDs. (If not, you get zero.)
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>quote</CODE
> <TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
></DT
><DD
><P
> Doubles all occurrences of single quote and backslash characters
in the given string. This can be used to safely quote strings
that are to be inserted into SQL commands given
to <CODE
CLASS="FUNCTION"
>spi_exec</CODE
> or
<CODE
CLASS="FUNCTION"
>spi_prepare</CODE
>.
For example, think about an SQL command string like:
</P><PRE
CLASS="PROGRAMLISTING"
>"SELECT '$val' AS ret"</PRE
><P>
where the Tcl variable <TT
CLASS="LITERAL"
>val</TT
> actually contains
<TT
CLASS="LITERAL"
>doesn't</TT
>. 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
><CODE
CLASS="FUNCTION"
>elog</CODE
> <TT
CLASS="REPLACEABLE"
><I
>level</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>msg</I
></TT
></DT
><DD
><P
> Emits a log or error message. Possible levels are
<TT
CLASS="LITERAL"
>DEBUG</TT
>, <TT
CLASS="LITERAL"
>LOG</TT
>, <TT
CLASS="LITERAL"
>INFO</TT
>,
<TT
CLASS="LITERAL"
>NOTICE</TT
>, <TT
CLASS="LITERAL"
>WARNING</TT
>, <TT
CLASS="LITERAL"
>ERROR</TT
>, and
<TT
CLASS="LITERAL"
>FATAL</TT
>. <TT
CLASS="LITERAL"
>ERROR</TT
>
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 <TT
CLASS="LITERAL"
>error</TT
> command.
<TT
CLASS="LITERAL"
>FATAL</TT
> 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
HREF="runtime-config-logging.html#GUC-LOG-MIN-MESSAGES"
>log_min_messages</A
> and
<A
HREF="runtime-config-logging.html#GUC-CLIENT-MIN-MESSAGES"
>client_min_messages</A
> configuration
variables. See <A
HREF="runtime-config.html"
>Chapter 18</A
> for more
information.
</P
></DD
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