## PL/SQL RPN calculator

Some non-XML stuff for a change :)

This is something I’ve developed recently to evaluate stored expressions (formulas) using variables.

The “calculator” is written in PL/SQL and implements an RPN evaluation technique, as well as a method to convert infix expressions to RPN using the shunting-yard algorithm.

The evaluation function does **not** involve any dynamic SQL.

Program and additional objects available here : rpn_util.zip

#### 1. Package specification

Here’s the package spec exposing both parse and eval methods.

create or replace package rpn_util is function parse (p_expr in varchar2) return token_list deterministic ; function eval (tlist in token_list, vars in kv_table default kv_table()) return binary_double deterministic ; end rpn_util;

and the supporting object types :

create or replace type token_list is table of varchar2(30) ; create or replace type kv_obj is object ( token varchar2(30) , val binary_double ) ; create or replace type kv_table is table of kv_obj ;

#### 2. Examples

- The parse function

The function takes a VARCHAR2 string representing an infix-based expression and returns a collection of RPN “tokens” in a TOKEN_LIST instance :

SQL> select rpn_util.parse('(1+2)*3-5') as tokens 2 from dual; TOKENS ---------------------------------------------------------------- TOKEN_LIST('1', '2', '+', '3', '*', '5', '-')

SQL> select rpn_util.parse('(1+2)*3-)5') as tokens 2 from dual; select rpn_util.parse('(1+2)*3-)5') as tokens * ERROR at line 1: ORA-20002: Parentheses mismatched: at position 9 ORA-06512: at "DEV.RPN_UTIL", line 134

The function takes a TOKEN_LIST collection of RPN tokens and returns the result of the expression :

SQL> select rpn_util.eval( 2 rpn_util.parse('(1+2)*3-5') 3 ) as result 4 from dual; RESULT ------ 4

Besides standard arithmetic operators (+, -, *, /, % and ^), it is also possible to parse and evaluate functions.

Functions we want to use this way have to be declared in the package : identifier, argument count and actual PL/SQL implementation.

For example, MIN is internally mapped to SQL function LEAST and ABS (absolute value) to… ABS :

SQL> select rpn_util.eval( 2 rpn_util.parse('MIN(3,2) * ABS(4-5)') 3 ) as result 4 from dual; RESULT ------ 2

Comparison (=, <, <=, > and >=) and boolean (AND, OR, NOT) operators are also supported via the IF function :

SQL> select rpn_util.eval( 2 rpn_util.parse('(MIN(3,2) * ABS(4-5))^IF(V1>2 OR NOT(V2=1 AND V3>=2),3,2)') 3 , kv_table( 4 kv_obj('V1',1) 5 , kv_obj('V2',1) 6 , kv_obj('V3',1) 7 ) 8 ) as result 9 from dual ; RESULT ---------- 8

The function takes a second optional parameter of KV_TABLE datatype which is a collection of key-value pairs. Each item of the collection is a KV_OBJ object instance holding the variable name (token) referenced by the formula and its actual value.

The values are then “bound” to the formula during evaluation.

SQL> select rpn_util.eval( 2 rpn_util.parse('(A + B) * C - D') 3 , kv_table( 4 kv_obj('A', 1) 5 , kv_obj('B', 2) 6 , kv_obj('C', 3) 7 , kv_obj('D', 5) 8 ) 9 ) as result 10 from dual; RESULT ------ 4

My initial requirement was to evaluate a lot of formulas in one go, with multiple and different input values. Given this specific scenario, I was very reluctent to use dynamic (PL/)SQL – a classic method for the job nonetheless.

A typical (yet simple) use case is the calculation of a thermal efficiency *η = output/input*, where *input* and *output* would come from metering devices and would be stored in a dedicated table along with the timestamp of the measures.

The next section shows an example of this situation, with a more complex formula.

#### 3. Benchmarking

To run some comparison tests, here’s a simplified version of my measure table. It stores hourly data of seven variables (A to G) for year 2012 :

SQL> desc measure_fact Name Null? Type ----------------------------------------- -------- ---------------------------- DT DATE TOKEN VARCHAR2(30) VAL NUMBER SQL> select * from measure_fact order by dt, token ; DT TOKEN VAL ------------------- ---------- ---------- 01/01/2012 00:00:00 A ,581904481 01/01/2012 00:00:00 B ,161674702 01/01/2012 00:00:00 C ,75959276 01/01/2012 00:00:00 D ,524613253 01/01/2012 00:00:00 E ,25470387 01/01/2012 00:00:00 F ,723589015 01/01/2012 00:00:00 G ,06515539 01/01/2012 01:00:00 A ,418608775 01/01/2012 01:00:00 B ,367579034 01/01/2012 01:00:00 C ,579998936 01/01/2012 01:00:00 D ,982256601 01/01/2012 01:00:00 E ,549623049 01/01/2012 01:00:00 F ,074974691 01/01/2012 01:00:00 G ,57354791 <snip> 31/12/2012 23:00:00 A ,874009005 31/12/2012 23:00:00 B ,825065349 31/12/2012 23:00:00 C ,493866511 31/12/2012 23:00:00 D ,36120651 31/12/2012 23:00:00 E ,308018922 31/12/2012 23:00:00 F ,672652837 31/12/2012 23:00:00 G ,241310825 61488 rows selected.

For each hour of the year, I’ll calculate the formula *ABS( ( ( A + B ) * C^D ) – E )^( F / G )* with three different methods and compare them : dynamic SQL with literal replacement, dynamic SQL with bind variables and the RPN evaluation technique.

##### a. Using a dynamic PL/SQL block with literal replacement

The “dynamic_eval” function is defined as follows :

create or replace function dynamic_eval (expr in varchar2, vars in kv_table) return binary_double is tmp varchar2(4000) := expr; res binary_double := 0; begin for i in 1..vars.count loop tmp := replace(tmp, ':'||vars(i).token, vars(i).val); end loop; execute immediate 'begin :1 := '||tmp||'; end;' using out res; return res; end;

The function is very simple, it just replaces each occurrence of the token in the input string with its value and use native dynamic sql to evaluate the resulting numeric expression.

Use case :

SQL> set autotrace traceonly statistics SQL> set timing on SQL> select dt 2 , dynamic_eval('power(ABS(((:A+:B)*power(:C,:D))-:E),(:F/:G))' 3 , cast(collect(kv_obj(token, val)) as kv_table)) as res 4 from measure_fact m 5 group by m.dt; 8784 rows selected. Elapsed: 00:01:07.18 Statistics ---------------------------------------------------------- 35136 recursive calls 0 db block gets 316 consistent gets 0 physical reads 0 redo size 234721 bytes sent via SQL*Net to client 6854 bytes received via SQL*Net from client 587 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 8784 rows processed

The query is quite slow, for an obvious reason : it hard-parses 8764 different cursors!

##### b. Using a dynamic PL/SQL block with bind variables

The “dynamic_eval2″ function is defined as follows :

create or replace function dynamic_eval2 (expr in varchar2, vars in kv_table) return binary_double is tmp varchar2(4000) := expr; res binary_double := 0; c number := dbms_sql.open_cursor; dummy number; begin dbms_sql.parse(c, 'begin :0 := '||tmp||'; end;', dbms_sql.native); for i in 1..vars.count loop dbms_sql.bind_variable(c, vars(i).token, vars(i).val); end loop; dbms_sql.bind_variable(c, '0', res); dummy := dbms_sql.execute(c); dbms_sql.variable_value(c, '0', res); dbms_sql.close_cursor(c); return res; end;

This version is a little bit more complex than the previous one. It uses DBMS_SQL to parse the PL/SQL block and bind the values from the input collection to their respective tokens.

Use case :

SQL> select dt 2 , dynamic_eval2('power(ABS(((:A+:B)*power(:C,:D))-:E),(:F/:G))' 3 , cast(collect(kv_obj(token, val)) as kv_table)) as res 4 from measure_fact m 5 group by m.dt; 8784 rows selected. Elapsed: 00:00:12.79 Statistics ---------------------------------------------------------- 43920 recursive calls 0 db block gets 316 consistent gets 0 physical reads 0 redo size 234721 bytes sent via SQL*Net to client 6854 bytes received via SQL*Net from client 587 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 8784 rows processed

We can immediately measure the improvement due to the bind variables : the cursor is parsed once and executed multiple times. However, the large number of recursive calls is still problematic.

##### c. Using the RPN technique

SQL> select m.dt 2 , rpn_util.eval( 3 rpn_util.parse('ABS(((A+B)*C^D)-E)^(F/G)') 4 , cast( 5 collect(kv_obj(m.token, m.val)) 6 as kv_table 7 ) 8 ) as res 9 from measure_fact m 10 group by m.dt ; 8784 rows selected. Elapsed: 00:00:04.68 Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 316 consistent gets 0 physical reads 0 redo size 189000 bytes sent via SQL*Net to client 6854 bytes received via SQL*Net from client 587 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 8784 rows processed

No recursive call, and execution time reduced by almost 2/3.

*Note : the parse function is called only once because it’s been declared DETERMINISTIC.*

##### d. Conclusion

Method | Rows fetched | Overall time (second) |
---|---|---|

Dynamic SQL with literal replacement | 8784 | 67 |

Dynamic SQL with bind variables | 8784 | 12.8 |

PL/SQL RPN evaluation | 8784 | 4.7 |

Hi,

I am new XML, and wondering if you could guide me in thr right direction.

I have XML file, which has mutiple complex types. I have the XSD for the same. I need to read the XML file (Extract set of nodes based on the data from a oracle table / cursor) and create a XML file using the same XSD struture.

Example i have a bank data in xml format with an XSD

I have to extract data of certain bank accounts and create an xml file with the same structure as the original source.

What approach should i take in Oracle 11g database ?