44.5. Database Access from PL/Tcl

The following commands are available to access the database from the body of a PL/Tcl function:

spi_exec ?-count n? ?-array name? command ?loop-body? : Executes an SQL command given as a string. An error in the command causes an error to be raised. Otherwise, the return value of spi_exec 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 SELECT statement, the values of the selected columns are placed in Tcl variables as described below.

The optional `-count` value tells
`spi_exec` 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 `FETCH n`.

If the command is a `SELECT` statement, the values of the
result columns are placed into Tcl variables named after the columns.
If the `-array` 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 “`.tupno`”, unless that name is
in use as a column name in the result.

If the command is a `SELECT` statement and no **`loop-body`**
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 `spi_exec`.)
For example:

```programlisting
spi_exec "SELECT count(*) AS cnt FROM pg_proc"
```

will set the Tcl variable `$cnt` to the number of rows in
the `pg_proc` system catalog.

If the optional **`loop-body`** argument is given, it is
a piece of Tcl script that is executed once for each row in the
query result. (**`loop-body`** is ignored if the given
command is not a `SELECT`.)
The values of the current row's columns
are stored into Tcl variables or array elements before each iteration.
For example:

```programlisting
spi_exec -array C "SELECT * FROM pg_class" {
    elog DEBUG "have table $C(relname)"
}
```

will print a log message for every row of `pg_class`. This
feature works similarly to other Tcl looping constructs; in
particular `continue` and `break` work in the
usual way inside the loop body.

If a column of a query result is null, the target
variable for it is “unset” rather than being set.

spi_prepare query typelist : Prepares and saves a query plan for later execution. The saved plan will be retained for the life of the current session.

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 `$1` ... `$n`.
If the query uses parameters, the names of the parameter types
must be given as a Tcl list. (Write an empty list for
**`typelist`** if no parameters are used.)

The return value from `spi_prepare` is a query ID
to be used in subsequent calls to `spi_execp`. See
`spi_execp` for an example.

spi_execp ?-count n? ?-array name? ?-nulls string? queryid ?value-list? ?loop-body? : Executes a query previously prepared with spi_prepare. queryid is the ID returned by spi_prepare. If the query references parameters, a value-list 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 spi_prepare. Omit value-list if the query has no parameters.

The optional value for `-nulls` is a string of spaces and
`'n'` characters telling `spi_execp`
which of the parameters are null values. If given, it must have exactly the
same length as the **`value-list`**. If it
is not given, all the parameter values are nonnull.

Except for the way in which the query and its parameters are specified,
`spi_execp` works just like `spi_exec`.
The `-count`, `-array`, and
**`loop-body`** options are the same,
and so is the result value.

Here's an example of a PL/Tcl function using a prepared plan:

```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;
```

We need backslashes inside the query string given to
`spi_prepare` to ensure that the
`$n` markers will be passed
through to `spi_prepare` as-is, and not replaced by Tcl
variable substitution.

spi_lastoid : Returns the OID of the row inserted by the last spi_exec or spi_execp, if the command was a single-row INSERT and the modified table contained OIDs. (If not, you get zero.)

subtransaction command : The Tcl script contained in command is executed within a 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 Section 44.9 for more details and an example.

quote string : 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 spi_exec or spi_prepare. For example, think about an SQL command string like:

```programlisting
"SELECT '$val' AS ret"
```

where the Tcl variable `val` actually contains
`doesn't`. This would result
in the final command string:

```programlisting
SELECT 'doesn't' AS ret
```

which would cause a parse error during
`spi_exec` or
`spi_prepare`.
To work properly, the submitted command should contain:

```programlisting
SELECT 'doesn''t' AS ret
```

which can be formed in PL/Tcl using:

```programlisting
"SELECT '[ quote $val ]' AS ret"
```

One advantage of `spi_execp` 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.

elog level msg : Emits a log or error message. Possible levels are DEBUG, LOG, INFO, NOTICE, WARNING, ERROR, and FATAL. ERROR 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 error command. FATAL 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 log_min_messages and client_min_messages configuration variables. See Chapter 19 and Section 44.8 for more information.