考虑下面的 PL/SQL 代码，这段代码生成一个 XML 格式的矩阵样式的报表：
| 以下为引用的内容：
declare
l_count integer;
begin
dbms_output.put_line('<matrix>');
-- generate matrix of parts by country
for part in (select id,description from parts order by description) loop
dbms_output.put_line('<row>');
dbms_output.put_line('<cell>'part.description'</cell>');
for country in (select code from countries order by name) loop
select sum(cnt) into l_count from orders
where part_id = part.id and cc = country.code;
dbms_output.put_line('<cell>'nvl(l_count,0)'</cell>');
end loop;
dbms_output.put_line('</row>');
end loop;
dbms_output.put_line('</matrix>');
end;
|
假如在这个例子中 parts 和 countries 有很多行数据，那么性能就会趋于下降。这是因为，在 PL/SQL 中，每次碰到一个游标 FOR 循环，在重新查询并获得数据时，都会有一个切换到 SQL 的上下文切换。
以一些服务器端内存为代价，提高这种构造的速度是有可能做到的——假如动态构建 PL/SQL 数据表和矩阵单元格条目就可以提高速度。例如：
| 以下为引用的内容：
declare
type part_tbl_type is table of parts%rowtype index by binary_integer;
part_tbl part_tbl_type;
--
type country_tbl_type is table of countries%rowtype index by binary_integer;
country_tbl country_tbl_type;
--
type cell_rec is record
(
part_id orders.part_id%type,
cc orders.cc%type,
cnt orders.cnt%type
);
type cell_tbl_type is table of cell_rec index by binary_integer;
cell_tbl cell_tbl_type;
--
i pls_integer;
begin
-- build rows
for row in (select * from parts order by description) loop
part_tbl(part_tbl.count+1) := row;
end loop;
-- build columns
for col in (select * from countries order by name) loop
country_tbl(country_tbl.count+1) := col;
end loop;
-- build cells
for cell in (select part_id,cc,sum(cnt) from orders group by part_id,cc) loop
cell_tbl(cell_tbl.count+1) := cell;
end loop;
dbms_output.put_line('<matrix>');
-- generate matrix of parts by country
i := cell_tbl.first;
for row in part_tbl.first .. part_tbl.last loop
dbms_output.put_line('<row>');
dbms_output.put_line('<cell>'part_tbl(row).description'</cell>');
for col in country_tbl.first .. country_tbl.last loop
if cell_tbl(i).part_id = part_tbl(row).id
and cell_tbl(i).cc = country_tbl(col).code
then
dbms_output.put_line('<cell>'cell_tbl(i).cnt'</cell>');
i := i + 1;
else
dbms_output.put_line('<cell>0</cell>');
end if;
end loop;
dbms_output.put_line('</row>');
end loop;
dbms_output.put_line('</matrix>');
end;
|
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