Details
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Improvement
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Status: Open
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Major
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Resolution: Unresolved
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None
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Description
The "workCount" setting for temporary table sizes is not a good configuration option. Binding sizes could potentially vary from as little as 32 bytes (8 byte ref to the binding + 8 byte ref to a variable + 8 byte nodeID + 8 byte object overhead), to some bindings with multi-megabyte strings. Asking the user to know which one it is likely to be, and then how that count translates into memory usage (the real resource we are attempting to control) is already way too much IMO.
OK, so what the user wants is a way to specify the amount of memory that can be used by each query operator for temporary tables [1][2][3]. Hmm, wait, no what he maybe wants is a way to specify a the total memory used for temporary tables per query? No, maybe he wants to specify it for the whole query engine.
But that last paragraph is not accurate. What he really wants is a system that answers all of his queries for whatever data he has as fast as possible. He doesn't want to have to configure any parameters. Unfortunately, this is a really hard dynamic optimization problem so we foist it off on the user, hoping he'll be able to come up with some value.
We need to decide on what we want to use as a config parameter. I believe it should be a "workMem" or "tmpTableSize" setting that specifies the max memory usage of a temporary table before it is converted into an on-disk table.
[1] This is what most DB systems provide, specifically PostgreSQL and MySQL both have per operator temporary table sizes. PostgreSQL calls the setting "work_mem" and MySQL calls it "tmp_table_size"
[2] http://www.postgresql.org/docs/8.3/static/runtime-config-resource.html
[3] http://dev.mysql.com/doc/refman/5.0/en/internal-temporary-tables.html
Attachments
Issue Links
- is related to
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JENA-119 Eliminate memory bounds during query execution
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- Closed
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