Postgresql 中文操作指南

43.13. Porting from Oracle PL/SQL #

本部分说明了 PostgreSQL 的 PL/pgSQL 语言和 Oracle 的 PL/SQL 语言之间的区别，以帮助将应用程序从 Oracle® 移植到 PostgreSQL 的开发者。

This section explains differences between PostgreSQL’s PL/pgSQL language and Oracle’s PL/SQL language, to help developers who port applications from Oracle® to PostgreSQL.

PL/pgSQL 在许多方面与 PL/SQL 类似。它是一种块结构的命令式语言，所有变量都必须声明。赋值、循环和条件类似。从 PL/SQL 移植到 PL/pgSQL 时应记住的主要区别是：

PL/pgSQL is similar to PL/SQL in many aspects. It is a block-structured, imperative language, and all variables have to be declared. Assignments, loops, and conditionals are similar. The main differences you should keep in mind when porting from PL/SQL to PL/pgSQL are:

43.13.1. Porting Examples #

Example 43.9 展示如何将一个简单的函数从 PL/SQL 移植到 PL/pgSQL。

Example 43.9 shows how to port a simple function from PL/SQL to PL/pgSQL.

Example 43.9. Porting a Simple Function from PL/SQL to PL/pgSQL

下面是一个 Oracle PL/SQL 函数：

Here is an Oracle PL/SQL function:

CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar2,
                                                  v_version varchar2)
RETURN varchar2 IS
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
/
show errors;

让我们浏览此函数，了解与 PL/pgSQL 相比的区别：

Let’s go through this function and see the differences compared to PL/pgSQL:

将此函数移植到 PostgreSQL 后，它的外观如下：

This is how this function would look when ported to PostgreSQL:

CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
                                                  v_version varchar)
RETURNS varchar AS $$
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;

Example 43.10 展示如何移植一个创建其他函数的函数，以及如何处理随之而来的引用问题。

Example 43.10 shows how to port a function that creates another function and how to handle the ensuing quoting problems.

Example 43.10. Porting a Function that Creates Another Function from PL/SQL to PL/pgSQL

以下过程从 SELECT 语句获取行，并为结果构建一个大型函数，以提高效率。

The following procedure grabs rows from a SELECT statement and builds a large function with the results in IF statements, for the sake of efficiency.

这是 Oracle 版本：

This is the Oracle version:

CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
    CURSOR referrer_keys IS
        SELECT * FROM cs_referrer_keys
        ORDER BY try_order;
    func_cmd VARCHAR(4000);
BEGIN
    func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR2,
                 v_domain IN VARCHAR2, v_url IN VARCHAR2) RETURN VARCHAR2 IS BEGIN';

    FOR referrer_key IN referrer_keys LOOP
        func_cmd := func_cmd ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ''' || referrer_key.key_string
          || ''' THEN RETURN ''' || referrer_key.referrer_type
          || '''; END IF;';
    END LOOP;

    func_cmd := func_cmd || ' RETURN NULL; END;';

    EXECUTE IMMEDIATE func_cmd;
END;
/
show errors;

下面是如何在 PostgreSQL 中实现该函数：

Here is how this function would end up in PostgreSQL:

CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc() AS $func$
DECLARE
    referrer_keys CURSOR IS
        SELECT * FROM cs_referrer_keys
        ORDER BY try_order;
    func_body text;
    func_cmd text;
BEGIN
    func_body := 'BEGIN';

    FOR referrer_key IN referrer_keys LOOP
        func_body := func_body ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ' || quote_literal(referrer_key.key_string)
          || ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
          || '; END IF;' ;
    END LOOP;

    func_body := func_body || ' RETURN NULL; END;';

    func_cmd :=
      'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
                                                        v_domain varchar,
                                                        v_url varchar)
        RETURNS varchar AS '
      || quote_literal(func_body)
      || ' LANGUAGE plpgsql;' ;

    EXECUTE func_cmd;
END;
$func$ LANGUAGE plpgsql;

请注意，函数体是单独构建并通过 quote_literal 传递以对其内的任何引号加倍。需要此技巧，因为我们无法安全使用美元符号引用来定义新函数：我们不确定将从 referrer_key.key_string 字段内插值哪些字符串。（我们在此假设可以始终信任 referrer_key.kind 为 host、domain 或 url，但 referrer_key.key_string 可能为任何内容，特别是，它可能包含美元符号。）这个函数实际上改进了原始 Oracle 函数，因为即使 referrer_key.key_string 或 referrer_key.referrer_type 包含引号，它也不会生成错误代码。

Notice how the body of the function is built separately and passed through quote_literal to double any quote marks in it. This technique is needed because we cannot safely use dollar quoting for defining the new function: we do not know for sure what strings will be interpolated from the referrer_key.key_string field. (We are assuming here that referrer_key.kind can be trusted to always be host, domain, or url, but referrer_key.key_string might be anything, in particular it might contain dollar signs.) This function is actually an improvement on the Oracle original, because it will not generate broken code when referrer_key.key_string or referrer_key.referrer_type contain quote marks.

Example 43.11 展示如何移植一个使用 OUT 参数和字符串操作的函数。PostgreSQL 没有一个内置的 instr 函数，但你可以使用其他函数的组合创建一个这样的函数。在 Section 43.13.3 中，有一个 PL/pgSQL 实现 instr，你可以使用它简化移植。

Example 43.11 shows how to port a function with OUT parameters and string manipulation. PostgreSQL does not have a built-in instr function, but you can create one using a combination of other functions. In Section 43.13.3 there is a PL/pgSQL implementation of instr that you can use to make your porting easier.

Example 43.11. Porting a Procedure With String Manipulation and OUT Parameters from PL/SQL to PL/pgSQL

以下 Oracle PL/SQL 过程用于解析 URL 并返回几个元素（主机、路径和查询）。

The following Oracle PL/SQL procedure is used to parse a URL and return several elements (host, path, and query).

这是 Oracle 版本：

This is the Oracle version:

CREATE OR REPLACE PROCEDURE cs_parse_url(
    v_url IN VARCHAR2,
    v_host OUT VARCHAR2,  -- This will be passed back
    v_path OUT VARCHAR2,  -- This one too
    v_query OUT VARCHAR2) -- And this one
IS
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;

以下可能翻译成 PL/pgSQL：

Here is a possible translation into PL/pgSQL:

CREATE OR REPLACE FUNCTION cs_parse_url(
    v_url IN VARCHAR,
    v_host OUT VARCHAR,  -- This will be passed back
    v_path OUT VARCHAR,  -- This one too
    v_query OUT VARCHAR) -- And this one
AS $$
DECLARE
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
$$ LANGUAGE plpgsql;

可以使用类似以下方式调用此函数：

This function could be used like this:

SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');

Example 43.12 展示如何移植一个使用许多特定于 Oracle 的特性的过程。

Example 43.12 shows how to port a procedure that uses numerous features that are specific to Oracle.

Example 43.12. Porting a Procedure from PL/SQL to PL/pgSQL

Oracle 版本：

The Oracle version:

CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
    a_running_job_count INTEGER;
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count > 0 THEN
        COMMIT; -- free lock
        raise_application_error(-20000,
                 'Unable to create a new job: a job is currently running.');
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
    EXCEPTION
        WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
    END;
    COMMIT;
END;
/
show errors

以下是移植此过程到 PL/pgSQL 的方法：

This is how we could port this procedure to PL/pgSQL:

CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id integer) AS $$
DECLARE
    a_running_job_count integer;
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count > 0 THEN
        COMMIT; -- free lock
        RAISE EXCEPTION 'Unable to create a new job: a job is currently running'; -- (1)
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
    EXCEPTION
        WHEN unique_violation THEN -- (2)
            -- don't worry if it already exists
    END;
    COMMIT;
END;
$$ LANGUAGE plpgsql;

43.13.2. Other Things to Watch For #

本部分说明了将 Oracle PL/SQL 函数移植到 PostgreSQL 时需要注意的其他一些事项。

This section explains a few other things to watch for when porting Oracle PL/SQL functions to PostgreSQL.

43.13.2.1. Implicit Rollback after Exceptions #

在 PL/pgSQL 中，当 EXCEPTION 子句捕获异常时，自该块的 BEGIN 起对数据库的所有更改都将自动回滚。也就是说，行为相当于你在 Oracle 中使用：

In PL/pgSQL, when an exception is caught by an EXCEPTION clause, all database changes since the block’s BEGIN are automatically rolled back. That is, the behavior is equivalent to what you’d get in Oracle with:

BEGIN
    SAVEPOINT s1;
    ... code here ...
EXCEPTION
    WHEN ... THEN
        ROLLBACK TO s1;
        ... code here ...
    WHEN ... THEN
        ROLLBACK TO s1;
        ... code here ...
END;

如果你正在翻译一个以这种样式使用 SAVEPOINT 和 ROLLBACK TO 的 Oracle 过程，你的任务很简单：省略 SAVEPOINT 和 ROLLBACK TO。如果你有一个以不同方式使用 SAVEPOINT 和 ROLLBACK TO 的过程，则需要考虑一些实际情况。

If you are translating an Oracle procedure that uses SAVEPOINT and ROLLBACK TO in this style, your task is easy: just omit the SAVEPOINT and ROLLBACK TO. If you have a procedure that uses SAVEPOINT and ROLLBACK TO in a different way then some actual thought will be required.

43.13.2.2. EXECUTE #

PL/pgSQL 版本的 EXECUTE 与 PL/SQL 版本类似，但你必须记住使用 quote_literal 和 quote_ident，如 Section 43.5.4 中所述。如果你不使用这些函数，EXECUTE 'SELECT * FROM $1'; 类型的结构将不可靠地运行。

The PL/pgSQL version of EXECUTE works similarly to the PL/SQL version, but you have to remember to use quote_literal and quote_ident as described in Section 43.5.4. Constructs of the type EXECUTE 'SELECT * FROM $1'; will not work reliably unless you use these functions.

43.13.2.3. Optimizing PL/pgSQL Functions #

PostgreSQL 提供两个函数创建修饰符以优化执行：“可变性”（函数是否始终在给定相同参数时返回相同结果）和“严格性”（函数是否在任何参数为空时返回空）。有关详细信息，请参阅 CREATE FUNCTION 参考页面。

PostgreSQL gives you two function creation modifiers to optimize execution: “volatility” (whether the function always returns the same result when given the same arguments) and “strictness” (whether the function returns null if any argument is null). Consult the CREATE FUNCTION reference page for details.

利用这些优化属性时，您的 CREATE FUNCTION 语句可能如下所示：

When making use of these optimization attributes, your CREATE FUNCTION statement might look something like this:

CREATE FUNCTION foo(...) RETURNS integer AS $$
...
$$ LANGUAGE plpgsql STRICT IMMUTABLE;

43.13.3. Appendix #

本部分包含一组与 Oracle 兼容的 instr 函数的代码，您可以使用这些函数简化端口转换工作。

This section contains the code for a set of Oracle-compatible instr functions that you can use to simplify your porting efforts.

--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1, string2 [, n [, m]])
-- where [] denotes optional parameters.
--
-- Search string1, beginning at the nth character, for the mth occurrence
-- of string2.  If n is negative, search backwards, starting at the abs(n)'th
-- character from the end of string1.
-- If n is not passed, assume 1 (search starts at first character).
-- If m is not passed, assume 1 (find first occurrence).
-- Returns starting index of string2 in string1, or 0 if string2 is not found.
--

CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
BEGIN
    RETURN instr($1, $2, 1);
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;


CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                      beg_index integer)
RETURNS integer AS $$
DECLARE
    pos integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    length integer;
    ss_length integer;
BEGIN
    IF beg_index > 0 THEN
        temp_str := substring(string FROM beg_index);
        pos := position(string_to_search_for IN temp_str);

        IF pos = 0 THEN
            RETURN 0;
        ELSE
            RETURN pos + beg_index - 1;
        END IF;
    ELSIF beg_index < 0 THEN
        ss_length := char_length(string_to_search_for);
        length := char_length(string);
        beg := length + 1 + beg_index;

        WHILE beg > 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            IF string_to_search_for = temp_str THEN
                RETURN beg;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    ELSE
        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;


CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                      beg_index integer, occur_index integer)
RETURNS integer AS $$
DECLARE
    pos integer NOT NULL DEFAULT 0;
    occur_number integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    i integer;
    length integer;
    ss_length integer;
BEGIN
    IF occur_index <= 0 THEN
        RAISE 'argument ''%'' is out of range', occur_index
          USING ERRCODE = '22003';
    END IF;

    IF beg_index > 0 THEN
        beg := beg_index - 1;
        FOR i IN 1..occur_index LOOP
            temp_str := substring(string FROM beg + 1);
            pos := position(string_to_search_for IN temp_str);
            IF pos = 0 THEN
                RETURN 0;
            END IF;
            beg := beg + pos;
        END LOOP;

        RETURN beg;
    ELSIF beg_index < 0 THEN
        ss_length := char_length(string_to_search_for);
        length := char_length(string);
        beg := length + 1 + beg_index;

        WHILE beg > 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            IF string_to_search_for = temp_str THEN
                occur_number := occur_number + 1;
                IF occur_number = occur_index THEN
                    RETURN beg;
                END IF;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    ELSE
        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;