Numeric strings are numbers that are stored in text columns. Although it is a very bad practice, it does not automatically render an index useless if you consistently treat it as string:
SELECT ... FROM ... WHERE numeric_string = '42'
Of course this statement can use an index on
NUMERIC_STRING. If you compare it using a number, however, the database can no longer use this condition as an access predicate.
SELECT ... FROM ... WHERE numeric_string = 42
Note the missing quotes. Although some database yield an error (e.g. PostgreSQL) many databases just add an implicit type conversion.
SELECT ... FROM ... WHERE TO_NUMBER(numeric_string) = 42
It is the same problem as before. An index on
NUMERIC_STRING cannot be used due to the function call. The solution is also the same as before: do not convert the table column, instead convert the search term.
SELECT ... FROM ... WHERE numeric_string = TO_CHAR(42)
You might wonder why the database does not do it this way automatically? It is because converting a string to a number always gives an unambiguous result. This is not true the other way around. A number, formatted as text, can contain spaces, punctation, and leading zeros. A single value can be written in many ways:
42 042 0042 00042 ...
The database cannot know the number format used in the
NUMERIC_STRING column so it does it the other way around: the database converts the strings to numbers—this is an unambiguous transformation.
TO_CHAR function returns only one string representation of the number. It will therefore only match the first of above listed strings. If we use
TO_NUMBER, it matches all of them. That means there is not only a performance difference between the two variants but also a semantic difference!
Using numeric strings is generally troublesome: most importantly it causes performance problems due to the implicit conversion and also introduces a risk of running into conversion errors due to invalid numbers. Even the most trivial query that does not use any functions in the where clause can cause an abort with a conversion error if there is just one invalid number stored in the table.
Use numeric types to store numbers.
Note that the problem does not exist the other way around:
SELECT ... FROM ... WHERE numeric_number = '42'
The database will consistently transform the string into a number. It does not apply a function on the potentially indexed column: a regular index will therefore work. Nevertheless it is possible to do a manual conversion the wrong way:
SELECT ... FROM ... WHERE TO_CHAR(numeric_number) = '42'