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tmp table with blob not using disk

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  • tmp table with blob not using disk

    I have a 10 million record table that contain 3 blobs.
    We do not search on these blobs. Instead we use them to do calculations using custom udf functions that do summarization on the blobs.

    Anyway during large queries we see the 'copying to tmp' when we do some of these large queries. However when I do a show status I get this:
    mysql> show status like '%tmp%';
    +-------------------------+-------+
    | Variable_name | Value |
    +-------------------------+-------+
    | Created_tmp_disk_tables | 0 |
    | Created_tmp_files | 5 |
    | Created_tmp_tables | 1 |

    But I thought that blobs would force the use of tmp_disk_tables?

    Would performance benefit if I used memory disks as my tmpdir?

  • #2
    Are you doing SHOW GLOBAL STATUS, or just SHOW STATUS?

    About performance -- you'd have to determine that disk access was the bottleneck before you'd be able to know that.

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    • #3
      But I thought that blobs would force the use of tmp_disk_tables?

      From chapter 8 of MySQL performance tuning: If BLOB/TEXT fields are requested in the statement executed, disk based temporary tables are automatic.

      I think you skirt that by using custom UDF functions.

      Glenn

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      • #4
        Thanks. It was the missing GLOBAL option.

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