避免在 PL/SQL 中使用嵌套游標查詢
2024-07-21 02:34:39
供稿：網友
考慮下面的 PL/SQL 代碼，這段代碼生成一個 xml 格式的矩陣樣式的報表：
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;
假如在這個例子中 parts 和 countries 有很多行數據，那么性能就會趨于下降。這是因為，在 PL/SQL 中，每次碰到一個游標 FOR 循環，在重新查詢并獲得數據時，都會有一個切換到 SQL 的上下文切換。
以一些服務器端內存為代價，提高這種構造的速度是有可能做到的——假如動態構建 PL/SQL 數據表和矩陣單元格條目就可以提高速度。例如：
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;
游標
游標的 FOR 循環現在是獨立運行的，并且特定記錄、特定字段、特定單元格的數據被拷貝到三個 PL/SQL 表中。
然后利用記錄和字段具有特定順序這一事實，將結果構建到一個 PL/SQL 表的矩陣中。由于 GROUP BY 的隱式 SORT/MERGE 操作，單元格具有同樣的順序。單元格查詢已經被減少到一個查詢，替代了原來的矩陣每個單元格使用一個查詢。
假如字段的數目相當小，那么我們可以使用 BULK COLLECT 構建表。BULK COLLECT 不答應表記錄的填充，所以我們就需要為用于這個操作的每一列數據創建一個獨立的表。前面的例子可以采用 BULK COLLECT 重寫為另外一種形式。
declare
type part_id_tbl_type is table of parts.id%type;
type part_desc_tbl_type is table of parts.description%type;
part_id_tbl part_id_tbl_type;
part_desc_tbl part_desc_tbl_type;
--
type country_code_tbl_type is table of countries.code%type;
country_code_tbl country_code_tbl_type;
--
type cell_cnt_tbl_type is table of orders.cnt%type;
cell_part_id_tbl part_id_tbl_type;
cell_country_tbl country_code_tbl_type;
cell_cnt_tbl cell_cnt_tbl_type;
--
i pls_integer;
begin
-- gather rows
select id,description
bulk collect into part_id_tbl,part_desc_tbl
from parts
order by description;
-- gather columns
select code
bulk collect into country_code_tbl
from countries
order by name;
-- gather cells
select part_id,cc,sum(cnt)
bulk collect into cell_part_id_tbl,cell_country_tbl,cell_cnt_tbl
from orders
group by part_id,cc;
dbms_output.put_line('<matrix>');
-- generate matrix of parts by country
i := cell_cnt_tbl.first;
for row in part_id_tbl.first .. part_id_tbl.last loop
dbms_output.put_line('<row>');
dbms_output.put_line('<cell>'part_desc_tbl(row)'</cell>');
for col in country_code_tbl.first .. country_code_tbl.last loop
if cell_part_id_tbl(i) = part_id_tbl(row)
and cell_country_tbl(i) = country_code_tbl(col)
then
dbms_output.put_line('<cell>'cell_cnt_tbl(i)'</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;
