SQL Problems Requiring Cursors

From Seo Wiki - Search Engine Optimization and Programming Languages

Jump to: navigation, search

A cursor is a construct available in most implementations of SQL that allows the programmer to handle data in a row-by-row manner.

The trouble with cursor processing is that the row-by-row nature cannot easily be made to occur in parallel by database optimizers. This results in a failure of the cursor processing to take full advantage of the processing power available on most database platforms.

To overcome this problem, cursor logic can often be converted into set-based SQL queries. Set-based SQL uses the syntax "SELECT...FROM..." with various clauses to perform the data manipulation work. Database optimizers have little trouble dividing such queries into parallel threads, thus fully utilizing the database hardware.

The resulting performance gain can be literal orders of magnitude: hours become minutes, and minutes become seconds.

Despite the tremendous performce gains from set-based SQL, it can be challenging to find the SQL that replaces fairly straigh-forward cursors.

This article discusses examples of such conversions.

Contents

Constraints

In this article, the following constraints apply:

  • The term "cursor" includes all cursor-like behavior. For example, using a loop in a shell script that loops across SQL queries or the output of SQL queries is cursor-like behavior and does not achieve the goal of true set-based processing within the database.
  • All set-based SQL must be ANSI SQL. A number of vendors provide some extremely powerful proprietary extensions. The goal is to avoid such extensions in favor of ANSI SQL.
  • The solution must be generalizable. In one or more examples below, specific values may be used for demonstration purposes, but any solution must scale to any number feasible within the power of the database software and machine resources.

Example: Insert rows based on a count in the table itself

Problem

The table below represents marbles. The four text columns represent four characteristics of marbles. Each characteristic has two values for a total of 16 types of marbles.

The "quantity" column represents how many of that marble we have. But instead of having a quantity, we want to have one row for each marble.

Thus, the target table would have the four text columns, and a total of 40 + 20 + 20 + 10 + ... + 10 + 5 = 270 rows.

Source table:
  QUANTITY TEXTURE    APPEARANCE SHAPE      COLOR
---------- ---------- ---------- ---------- -----
        40 smooth     shiny      round      blue
        20 smooth     shiny      warped     blue
        20 smooth     dull       round      blue
        10 smooth     dull       warped     blue
        20 rough      shiny      round      blue
        10 rough      shiny      warped     blue
        10 rough      dull       round      blue
         5 rough      dull       warped     blue
        40 rough      dull       warped     red
        20 rough      dull       round      red
        20 rough      shiny      warped     red
        10 rough      shiny      round      red
        20 smooth     dull       warped     red
        10 smooth     dull       round      red
        10 smooth     shiny      warped     red
         5 smooth     shiny      round      red
Table to generate:
TEXTURE    APPEARANCE SHAPE      COLOR
---------- ---------- ---------- -----
smooth     shiny      round      blue   -- 1
smooth     shiny      round      blue   -- 2
...                                     -- and so on
smooth     shiny      round      blue   -- 40
smooth     shiny      warped     blue   -- 1
smooth     shiny      warped     blue   -- 2
...                                     -- and so on
smooth     shiny      warped     blue   -- 20
...                                     -- and so on
smooth     shiny      round      red    -- 1
smooth     shiny      round      red    -- 2
smooth     shiny      round      red    -- 3
smooth     shiny      round      red    -- 4
smooth     shiny      round      red    -- 5

Solution in cursor form

Generating the target table with a cursor is fairly simple.

pre> declare

 cursor c is select * from marbles_seed;

begin

 for r in c loop
   for i in 1..r.quantity loop
           

000000000000r.co

lor_predicted,

       r.accuracy,
       r.coverage
     );
   end loop;
 end loop;

end; </pre>

Solution in ANSI set-based SQL

Solving the problem with ANSI SQL is a bit more code, but requires a bit more creative thought than the nested loop approach of cursors.

Number Table

To reach the solution requires an intermediate table. The table has one column of type NUMBER that has the values 0 to whatever number of rows is needed. For this discussion, we'll limit it to one million rows. The code is as follows:

<

Solution Core

Assume the source table above is named marbles_seed and the target table is named marbles. The code that generates the needed 270 rows is:

insert into marbles
select m.texture,
       m.appearance,
       m.shape,
       m.color_actual,
       m.hits,
       m.color_predicted,
       m.accuracy,
       m.coverage
  from marbles_seed m,
       numbers n
 where m.quantity > n.n
Personal tools

Served in 0.134 secs.