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 NULL keyword. All expressions are evaluated internally by executing a query
using the SPI manager. In the expression, 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 re-used 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';
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
PostgreSQL main parser knows
when preparing the plan for the INSERT, that the string
'now' should be interpreted as
timestamp because the target field 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 backend.
Needless to say that this isn't what the programmer wanted.
In the case of
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
curtime, the PL/pgSQL interpreter casts this string to
the timestamp type by calling the
timestamp_in() functions for the conversion.
So, the computed timestamp is updated on each execution as the
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 datatype 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.)
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