SingleStore Managed Service

Comparing mismatched datatypes

Using comparisons between mismatched datatypes may degrade query performance and may use unsafe type conversions which can yield undesirable query results. SingleStore DB will display a warning for queries with potentially problematic comparisons between mismatched datatypes in the EXPLAIN and information_schema records for the query.

These warnings do not necessarily indicate a problem, and you may have valid reasons for comparing different datatypes. But these type mismatches are flagged to help you identify potential problems that you may not be aware of.

When you see these warnings, you should check whether the datatypes are expected to be different. You may wish to consider changing the datatypes of the fields or values involved. You may also wish to consider adding explicit type conversion operations, such as the cast operators or functions like STR_TO_DATE which convert between different types.

These warnings can be seen in EXPLAIN <query>, information_schema.plancache.plan_warnings, information_schema.mv_queries.plan_warnings, and the PlanWarnings column of show plancache in textual form, as well as in EXPLAIN JSON <query>, information_schema.plancache.plan_info, and information_schema.mv_queries.plan_info 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.i = 123, where t.i is a string field. When a numeric value is compared to a string, the string is converted to a numeric value, e.g. the string '123' is converted to the number 123.

This comparison may be problematic for multiple reasons:

  • The mismatched datatypes 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 integer 123, so while the query may be intended to retrieve a single row with the specified id, this equality comparison may actually yield multiple rows whose id values all compare equal to 123.

  • The mismatched datatypes negatively impact 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 actually many possible string ids that match the number 123, 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.