MDEV-40018: SHOW SLAVE STATUS allocates ~1.7 MB per call due to overized VARCHAR columns

[plampio]

This patch reduces the memory footprint of SHOW SLAVE STATUS and some other information schema queries by changing the type of 13 columns from VARCHAR to BLOB.

SHOW SLAVE STATUS / SHOW ALL SLAVES STATUS / INFORMATION_SCHEMA.SLAVE_STATUS are all served from the ST_FIELD_INFO array slave_status_info[]. Thirteen of its columns (Replicate_Do_DB, Replicate_Ignore_DB, Replicate_Do_Table, Replicate_Ignore_Table, Replicate_Wild_Do_Table, Replicate_Wild_Ignore_Table, Replicate_Ignore_Server_Ids, Gtid_IO_Pos, Replicate_Do_Domain_Ids, Replicate_Ignore_Domain_Ids, Slave_SQL_Running_State, Replicate_Rewrite_DB, and Gtid_Slave_Pos) used the no-argument Varchar() helper, which defaults to MAX_FIELD_VARCHARLENGTH/3 characters. With utf8mb3, that reserves 65532 bytes per column in the temporary table's record buffer regardless of the actual value size, and the record is allocated twice, accounting for the reported ~1.7 MB allocated on every call.

Change these columns to Blob(MAX_FIELD_VARCHARLENGTH), which keeps the same maximum capacity but stores only a pointer and length in the record. Values, NULL-ability, and column order are unchanged; the client-visible column type for these columns changes from VARCHAR to BLOB.

mysql-test/suite/funcs_1/r/is_columns_is.result is updated to match, since it asserts on INFORMATION_SCHEMA.COLUMNS metadata for these columns.

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