postgreSQL PL/SQL编程学习笔记（六）——杂七杂八

1 PL/pgSQL Under the Hood

This part discusses some implementation details that are frequently important for PL/pgSQL users to know.

1.1 Variable Substitution

SQL statements and expressions within a PL/pgSQL function can refer to variables and parameters of the function. But parameters will only be substituted in places where a parameter or column reference is syntactically allowed.

As an extreme case, consider this example of poor programming style:

INSERT INTO foo (foo) VALUES (foo);

The first occurrence of foo must syntactically be a table name, so it will not be substituted, even if the function has a variable named foo.

The second occurrence must be the name of a column of the table, so it will not be substituted either.

Only the third occurrence is a candidate to be a reference to the function’s variable.

If we change the previous example to

INSERT INTO dest (col) SELECT foo + bar FROM src;

Here, dest and src must be table names, and col must be a column of dest, but foo and bar might reasonably be either variables of the function or columns of src.

By default, PL/pgSQL will report an error if a name in a SQL statement could refer to either a variable or a table column. You can fix such a problem by renaming the variable or column, or by qualifying the ambiguous reference, or by telling PL/pgSQL which interpretation to prefer.

Alternatively you can qualify ambiguous references to make them clear. In the above example, src.foo would be an unambiguous reference to the table column. To create an unambiguous reference to a variable, declare it in a labeled block and use the block’s label. For example,

<<block>>
DECLARE
foo int;
BEGIN
foo := ...;
INSERT INTO dest (col) SELECT block.foo + bar FROM src;

Here block.foo means the variable even if there is a column foo in src. Function parameters, as well as special variables such as FOUND, can be qualified by the function’s name, because they are implicitly declared in an outer block labeled with the function’s name.

You can specify that PL/pgSQL should resolve ambiguous references as the variableor as the table column.

To change this behavior on a system-wide basis, set the configuration parameter plpgsql.variable_conflict to one of error, use_variable, or use_column (where error is the factory default). This parameter affects subsequent compilations of statements in PL/pgSQL functions, but not statements already compiled in the current session. it can only be changed by a superuser.

You can also set the behavior on a function-by-function basis, by inserting one of these special commands at the start of the function text:

#variable_conflict error
#variable_conflict use_variable
#variable_conflict use_column

These commands affect only the function they are written in, and override the setting of plpgsql.variable_conflict. An example is

CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
#variable_conflict use_variable
DECLARE
curtime timestamp := now();
BEGIN
UPDATE users SET last_modified = curtime, comment = comment
WHERE users.id = id;
END;
$$ LANGUAGE plpgsql;

In the UPDATE command, curtime, comment, and id will refer to the function’s variable and parameters whether or not users has columns of those names. Notice that we had to qualify the reference to users.id in the WHERE clause to make it refer to the table column. But we did not have to qualify the reference to comment as a target in the UPDATE list, because syntactically that must be a column of users.We could write the same function without depending on the variable_conflict setting in this way:

CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
<<fn>>
DECLARE
curtime timestamp := now();
BEGIN
UPDATE users SET last_modified = fn.curtime, comment = stamp_user.comment
WHERE users.id = stamp_user.id;
END;
$$ LANGUAGE plpgsql;

Variable substitution does not happen in the command string given to EXECUTE or one of its variants. If you need to insert a varying value into such a command, do so as part of constructing the string value, or use USING.

Variable substitution currently works only in SELECT, INSERT, UPDATE, and DELETE commands, because the main SQL engine allows query parameters only in these commands. To use a non-constant name or value in other statement types (generically called utility statements), you must construct the utility statement as a string and EXECUTE it.

1.2 Plan Caching

The PL/pgSQL interpreter parses the function’s source text and produces an internal binary instruction tree the first time the function is called (within each session). The instruction tree fully translates the PL/pgSQL statement structure, but individual SQL expressions and SQL commands used in the function are not translated immediately.

As each expression and SQL command is first executed in the function, the PL/pgSQL interpreter parses and analyzes the command to create a prepared statement, using the SPI manager’s SPI_prepare function. Subsequent visits to that expression or command reuse the prepared statement. Thus, a function with conditional code paths that are seldom visited will never incur the overhead of analyzing those commands that are never executed within the current session. A disadvantage is that errors in a specific expression or command cannot be detected until that part of the function is reached in execution. (Trivial syntax errors will be detected during the initial parsing pass, but anything deeper will not be detected until execution.)

PL/pgSQL (or more precisely, the SPI manager) can furthermore attempt to cache the execution plan associated with any particular prepared statement. If a cached plan is not used, then a fresh execution plan is generated on each visit to the statement, and the current parameter values (that is, PL/pgSQL variable values) can be used to optimize the selected plan. If the statement has no parameters, or is executed many times, the SPI manager will consider creating a generic plan that is not dependent on specific parameter values, and caching that for re-use. Typically this will happen only if the execution plan is not very sensitive to the values of the PL/pgSQL variables referenced in it. If it is, generating a plan each time is a net win. See PREPARE for more information about the behavior of prepared statements.

Because PL/pgSQL saves prepared statements and sometimes execution plans in this way, SQL commands that appear directly in a PL/pgSQL function must refer to the same tables and columns on every execution; that is, you cannot use a parameter as the name of a table or column in an SQL command. To get around this restriction, you can construct dynamic commands using the PL/pgSQL EXECUTE statement--at the price of performing new parse analysis and constructing a new execution plan on every execution.

The mutable nature of record variables presents another 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 from one call of the function to the next, since each expression will be analyzed using the data type that is present when the expression is first reached. EXECUTE can be used to get around this problem when necessary.

If the same function is used as a trigger for more than one table, PL/pgSQL prepares and caches statements independently for each such table -- that is, there is a cache for each trigger function and table combination, not just for each function. This alleviates some of the problems with varying data types; for instance, a trigger function will be able to work successfully with a column named key even if it happens to have different types in different tables.

Likewise, functions having polymorphic argument types have a separate statement cache for each combination of actual argument types they have been invoked for, so that data type differences do not cause unexpected failures.

Statement caching can sometimes have surprising effects on the interpretation of time-sensitive values. For example there is a difference between what these two functions do:

CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
BEGIN
INSERT INTO logtable VALUES (logtxt, ’now’);
END;
$$ LANGUAGE plpgsql;
and:
CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp;
BEGIN
curtime := ’now’;
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;

In the case of logfunc1, the PostgreSQL main parser knows when analyzing the INSERT that the string ’now’ should be interpreted as timestamp, because the target column of logtable is of that type. Thus, ’now’ will be converted to a timestamp constant when the INSERT is analyzed, and then used in all invocations of logfunc1 during the lifetime of the session. Needless to say, this isn’t what the programmer wanted. A better idea is to use the now() or current_timestamp function. 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. Even though this happens to work as expected, it’s not terribly efficient, so use of the now() function would still be a better idea.

2 Tips for Developing in PL/pgSQL

One good way to develop in PL/pgSQL is to use the text editor of your choice to create your functions, and in another window, use psql to load and test those functions. If you are doing it this way, it is a good idea to write the function using CREATE OR REPLACE FUNCTION. That way you can just reload the file to update the function definition. For example:

CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
....
$$ LANGUAGE plpgsql;

While running psql, you can load or reload such a function definition file with:

\i filename.sql

and then immediately issue SQL commands to test the function. Another good way to develop in PL/pgSQL is with a GUI database access tool that facilitates development in a procedural language. One example of such a tool is pgAdmin, although others exist. These tools often provide convenient features such as escaping single quotes and making it easier to recreate and debug functions.

2.1 Handling of Quotation Marks

The code of a PL/pgSQL function is specified in CREATE FUNCTION as a string literal. If you write the string literal in the ordinary way with surrounding single quotes, then any single quotes inside the function body must be doubled; likewise any backslashes must be doubled (assuming escape string syntax is used). Doubling quotes is at best tedious, and in more complicated cases the code can become downright incomprehensible, because you can easily find yourself needing half a dozen or more adjacent quote marks. It’s recommended that you instead write the function body as a “dollarquoted” string literal (see Section 4.1.2.4). In the dollar-quoting approach, you never double any quote marks, but instead take care to choose a different dollar-quoting delimiter for each level of nesting you need. For example, you might write the CREATE FUNCTION command as:

CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
....
$PROC$ LANGUAGE plpgsql;

Within this, you might use quote marks for simple literal strings in SQL commands and $$ to delimit fragments of SQL commands that you are assembling as strings. If you need to quote text that includes $$, you could use $Q$, and so on.

The following chart shows what you have to do when writing quote marks without dollar quoting. It might be useful when translating pre-dollar quoting code into something more comprehensible.

1 quotation mark

To begin and end the function body, for example:

CREATE FUNCTION foo() RETURNS integer AS ’
....
’ LANGUAGE plpgsql;

Anywhere within a single-quoted function body, quote marks must appear in pairs.

2 quotation marks

For string literals inside the function body, for example:

a_output := ”Blah”;
SELECT * FROM users WHERE f_name=”foobar”;

In the dollar-quoting approach, you’d just write:

a_output := ’Blah’;
SELECT * FROM users WHERE f_name=’foobar’;

which is exactly what the PL/pgSQL parser would see in either case.

4 quotation marks

When you need a single quotation mark in a string constant inside the function body, for example:

a_output := a_output || ” AND name LIKE ””foobar”” AND xyz”

The value actually appended to a_output would be: AND name LIKE ’foobar’ AND xyz.
In the dollar-quoting approach, you’d write:

a_output := a_output || $$ AND name LIKE ’foobar’ AND xyz$$

being careful that any dollar-quote delimiters around this are not just $$.

6 quotation marks

When a single quotation mark in a string inside the function body is adjacent to the end of that string constant, for example:

a_output := a_output || ” AND name LIKE ””foobar”””

The value appended to a_output would then be: AND name LIKE ’foobar’.

In the dollar-quoting approach, this becomes:

a_output := a_output || $$ AND name LIKE ’foobar’$$

10 quotation marks

When you want two single quotation marks in a string constant (which accounts for 8 quotation marks) and this is adjacent to the end of that string constant (2 more). You will probably only need that if you are writing a function that generates other functions, as in Example 41-9. For example:

a_output := a_output || ” if v_” ||
referrer_keys.kind || ” like ”””””
|| referrer_keys.key_string || ”””””
then return ””” || referrer_keys.referrer_type
|| ”””; end if;”;

The value of a_output would then be:

if v_... like ”...” then return ”...”; end if;

In the dollar-quoting approach, this becomes:

a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like ’$$
|| referrer_keys.key_string || $$’
then return ’$$ || referrer_keys.referrer_type
|| $$’; end if;$$;

where we assume we only need to put single quote marks into a_output, because it will be
re-quoted before use.

2.2 Additional Compile-time Checks

To aid the user in finding instances of simple but common problems before they cause harm, PL/PgSQL provides additional checks. When enabled, depending on the configuration, they can be used to emit either a WARNING or an ERROR during the compilation of a function. A function which has received a WARNING can be executed without producing further messages, so you are advised to test in a separate development environment.

These additional checks are enabled through the configuration variables plpgsql.extra_warnings for warnings and plpgsql.extra_errors for errors. Both can be set either to a comma-separated list of checks, "none" or "all". The default is "none". Currently the list of available checks includes only one:

shadowed_variables

Checks if a declaration shadows a previously defined variable.

The following example shows the effect of plpgsql.extra_warnings set to
shadowed_variables:

SET plpgsql.extra_warnings TO ’shadowed_variables’;
CREATE FUNCTION foo(f1 int) RETURNS int AS $$
DECLARE
f1 int;
BEGIN
RETURN f1;
END
$$ LANGUAGE plpgsql;
WARNING: variable "f1" shadows a previously defined variable
LINE 3: f1 int;
^
CREATE FUNCTION