Notes on BitExpression and Partitions

[AnyhowStep]

The following are semantically equivalent,

CREATE TABLE T (X INT)
  PARTITION BY HASH(X IS NULL);

CREATE TABLE T (X INT)
  PARTITION BY HASH((X IS NULL));

The difference is X IS NULL vs (X IS NULL).

When given X IS NULL, we get a syntax error.

When given (X IS NULL), we are told IS NULL is not a valid partition function (ER_PARTITION_FUNCTION_IS_NOT_ALLOWED) per,

• https://dev.mysql.com/doc/refman/5.7/en/partitioning-limitations-functions.html
• https://github.com/mysql/mysql-server/blob/3e90d07c3578e4da39dc1bce73559bbdf655c28c/sql/sql_partition.cc#L1423
• check_partition_func_processor https://github.com/mysql/mysql-server/search?q=check_partition_func_processor
  • Just walk over the expression node and follow check_partition_func_processor. If it returns false, we can use it. Otherwise, we can't

We should probably allow Expression in the HashPartition rule, then push error checks down the pipeline.

Right now, X IS NULL will give,

• Unexpected X
• Unexpected IS
• HASH(NULL)

And (X IS NULL) will just parse successfully without any parse errors.