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
? -countn
? ? -arrayname
?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 aSELECT
statement, the values of the selected columns are placed in Tcl variables as described below.The optional
-count
value tellsspi_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 sayingFETCH
.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 noloop-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 ofspi_exec
.) For example:spi_exec "SELECT count(*) AS cnt FROM pg_proc"
will set the Tcl variable
$cnt
to the number of rows in thepg_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 aSELECT
.) The values of the current row's columns are stored into Tcl variables or array elements before each iteration. For example: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 particularcontinue
andbreak
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
$1
...$
. If the query uses parameters, the names of the parameter types must be given as a Tcl list. (Write an empty list forn
typelist
if no parameters are used.)The return value from
spi_prepare
is a query ID to be used in subsequent calls tospi_execp
. Seespi_execp
for an example. -
spi_execp
? -countn
? ? -arrayname
? ? -nullsstring
?queryid
?value-list
? ?loop-body
? -
Executes a query previously prepared with
spi_prepare
.queryid
is the ID returned byspi_prepare
. If the query references parameters, avalue-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 tospi_prepare
. Omitvalue-list
if the query has no parameters.The optional value for
-nulls
is a string of spaces and'n'
characters tellingspi_execp
which of the parameters are null values. If given, it must have exactly the same length as thevalue-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 likespi_exec
. The-count
,-array
, andloop-body
options are the same, and so is the result value.Here's an example of a PL/Tcl function using a prepared plan:
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$
markers will be passed through ton
spi_prepare
as-is, and not replaced by Tcl variable substitution. -
subtransaction
command
-
The Tcl script contained in
command
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 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
orspi_prepare
. For example, think about an SQL command string like:"SELECT '$val' AS ret"
where the Tcl variable
val
actually containsdoesn't
. This would result in the final command string:SELECT 'doesn't' AS ret
which would cause a parse error during
spi_exec
orspi_prepare
. To work properly, the submitted command should contain:SELECT 'doesn''t' AS ret
which can be formed in PL/Tcl using:
"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
, andFATAL
.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 Tclerror
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 20 and Section 44.8 for more information.