![]() ![]() Pretty fast, but iterating on the replacements is very costly. Concatenating NULL values with non-NULL characters results in that character in Oracle, but NULL in PostgreSQL. Oracle reads empty strings as NULLs, while PostgreSQL treats them as empty. In a plpgsql implementation, searching with the alternation appears to be SUMMARY: This article discusses the differences between how Oracle and PostgreSQL evaluate NULL characters and empty strings. (tested up to 1000, knowing that they get combined into a single regularĮxpression) and many actual replacements occurring. This Perl function appears to be very fast, even with many strings to replace These fiveĬREATE FUNCTION multi_replace ( string text, orig text, repl text ) RETURNS text AS $BODY $ my ( $string, $orig, $repl ) = my %subs if ( != ) /g return $string $BODY $ language plperl strict immutable ![]() Job, and it seems the popular answer when searching forĪ solution on the web, but in the general case, they produce flawed results ifĪny inner replacement happens to create a match for other outerĪs an example, consider the necessary conversion of characters into HTMLĮntities when creating an HTML fragment from a raw text. In some cases, nested or successive replace() calls might do the Regexp_replace(string, 'foo|bar', 'baz', 'g') would replace bothįoo and bar by baz, but it cannot be instructed to replace foo by Multiple strings in a single pass, but it can’t do multiple In PostgreSQL, regexp_replace does support alternations to search for Or the substitutions operators with regular expressions in Python or Perl Multiple strings each by its own independent replacement, like for In EDB Postgres Advanced Server, there is a compatibility setting called edb_redwood_strings, which, when set to true, enables the same behaviour as Oracle when it comes to concatenation.Select replace('the string is bar', 'bar', 'foo') īut there isn’t a PostgreSQL core function to substitute What is interesting here is that, in Oracle, concatenating a NULL and a character together results in the character as the output value, whereas in PostgreSQL, the fact that a NULL is present in either value means that the result is NULL as the output value, no matter what it is being concatenated with. PostgreSQL: | id | content | concatnull | concatchar | Oracle: | ID | CONTENT | CONCATNULL | CONCATCHAR | The query we will be using is: SELECT id, content, Let's see what we get if we try concatenating a NULL or a 1-character string to the values in our example table. NULLs and non-NULLsĪnother important difference between Oracle and PostgreSQL is when a NULL value is concatenated with a non-NULL character. PostgreSQL's behaviour follows the standard in its treatment of NULL values. No conversion has occurred, and we can see that it isn't considered to be a NULL in the query results, but an empty string. But when we look at the empty string that we inserted for the 2nd row, we don't have a NULL value, we still have an empty string. If we look at the first two, the NULL we inserted is still considered a NULL and can't be compared to an empty string. We can ignore the bottom two rows because the functionality is the same, as expected. We don't need to change anything about the above DDL, DML or SQL, so let's just look at the results we end up with: | id | content | isnull | isempty | blank | Let's do the same thing again, but in PostgreSQL this time. NULLs and empty strings in PostgreSQLīut in PostgreSQL, the story is different. The same goes for when we have any non-whitespace characters it's all the same. However, if we have a single space, this isn't converted, as it isn't an empty string. So, empty strings cannot be stored in the database. This tells us that the empty string was treated as a NULL when inserted into the table, and that it can't be compared to regular values as if it were an empty string because it's a full-fledged NULL. | ID | CONTENT | ISNULL | ISEMPTY | BLANK | Note: Remember to change the default null output from '' to (null) psql -P 'null=(null)' SELECTĬASE WHEN content IS NULL THEN 1 ELSE 0 END AS isnull,ĬASE WHEN content = '' THEN 1 ELSE 0 END AS isempty,ĬASE WHEN content = ' ' THEN 1 ELSE 0 END AS blank Here we have a value that's explicitly NULL, an empty string, a string with a single space in it, and another with a 1-character string. INSERT INTO test (id, content) VALUES (4, 'x') INSERT INTO test (id, content) VALUES (3, ' ') INSERT INTO test (id, content) VALUES (2, '') INSERT INTO test (id, content) VALUES (1, NULL) We'll demonstrate this behaviour with a simple table and some data: CREATE TABLE test ( In Oracle, NULLs and empty strings are equivalent when it comes to values stored in the database. Oracle and PostgreSQL behave similarly in many cases, but one way they differ is in their treatment of NULLs and empty strings. SUMMARY: This article discusses the differences between how Oracle and PostgreSQL evaluate NULL characters and empty strings.
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