PostgreSQL 7.4.3 Documentation | ||||
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All expressions used in PL/pgSQL statements are processed using the server's regular SQL executor. Expressions that appear to contain constants may in fact require run-time evaluation (e.g., 'now' for the timestamp type) so it is impossible for the PL/pgSQL parser to identify real constant values other than the key word NULL. All expressions are evaluated internally by executing a query
SELECT expression
using the SPI manager. For evaluation, occurrences of PL/pgSQL variable identifiers are replaced by parameters, and the actual values from the variables are passed to the executor in the parameter array. This allows the query plan for the SELECT to be prepared just once and then reused for subsequent evaluations.
The evaluation done by the PostgreSQL main parser has some side effects on the interpretation of constant values. In detail there is a difference between what these two functions do:
CREATE FUNCTION logfunc1(text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; BEGIN INSERT INTO logtable VALUES (logtxt, ''now''); RETURN ''now''; END; ' LANGUAGE plpgsql;
and
CREATE FUNCTION logfunc2(text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; curtime timestamp; BEGIN curtime := ''now''; INSERT INTO logtable VALUES (logtxt, curtime); RETURN curtime; END; ' LANGUAGE plpgsql;
In the case of logfunc1
, the
PostgreSQL main parser knows when
preparing the plan for the INSERT, that the string
'now' should be interpreted as
timestamp because the target column of logtable
is of that type. Thus, it will make a constant from it at this
time and this constant value is then used in all invocations of
logfunc1
during the lifetime of the
session. Needless to say that this isn't what the
programmer wanted.
In the case of logfunc2
, the
PostgreSQL main parser does not know
what type 'now' should become and therefore
it returns a data value of type text containing the string
now. During the ensuing assignment
to the local variable curtime, the
PL/pgSQL interpreter casts this
string to the timestamp type by calling the
text_out
and timestamp_in
functions for the conversion. So, the computed time stamp is updated
on each execution as the programmer expects.
The mutable nature of record variables presents a problem in this connection. When fields of a record variable are used in expressions or statements, the data types of the fields must not change between calls of one and the same expression, since the expression will be planned using the data type that is present when the expression is first reached. Keep this in mind when writing trigger procedures that handle events for more than one table. (EXECUTE can be used to get around this problem when necessary.)