Comparing Mismatched Datatypes
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Using comparisons between mismatched data types may degrade query performance and may use unsafe type conversions which can yield undesirable query results.EXPLAIN and information_ records for the query.
These warnings do not necessarily indicate a problem, and you may have valid reasons for comparing different data types.
When you see these warnings, you should check whether the data types are expected to be different.
These warnings can be seen in EXPLAIN <query>, information_, information_, and the PlanWarnings column of show plancache in textual form, as well as in EXPLAIN JSON <query>, information_, and information_ in JSON form.
Example
An example of these warnings is the following:
CREATE TABLE t (id VARCHAR(50), PRIMARY KEY (id));EXPLAIN SELECT * FROM t WHERE id = 123;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| EXPLAIN |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| WARNING: Comparisons between mismatched datatypes which may involve unsafe datatype conversions and/or degrade performance. Consider changing the datatypes, or adding explicit typecasts. See https://docs.singlestore.com/docs/mismatched-datatypes for more information. |
| |
| WARNING: Comparison between mismatched datatypes: (`t`.`id` = 123). Types 'varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL' vs 'bigint(20) NOT NULL'. |
| |
| Gather partitions:all alias:remote_0 |
| Project [t.id] |
| Filter [t.id = 123] |
| TableScan db.t, PRIMARY KEY (id) |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+In this example, the query involves the expression t., where t. is a string field.'123' is converted to the number 123.
This comparison may be problematic for multiple reasons:
-
The mismatched data types may indicate a mistake in how the query is written or how the table is defined.
The query may behave differently than intended and yield undesired results. For example, all of the strings '123','123.,0' '0123', and'123abc'compare equal to the integer123, so while the query may be intended to retrieve a single row with the specifiedid, this equality comparison may yield multiple rows whoseidvalues all compare equal to123. -
The mismatched data types negatively impact the performance of the query.
If the field and constant were either both strings or both integers, the query plan would be able to use the index to efficiently seek to lookup the matching id.But because there are many possible string ids that match the number123, which do not come in any particular order in terms of string lexicographic order, the query cannot seek into the index and instead must scan all the rows of the table.
Last modified: March 8, 2024