Updating avg from bin file

07-Apr-2020 09:15 by 2 Comments

Updating avg from bin file

The purpose is to ensure the optimizer selects the most efficient query plan for your queries. Normally stability is achieved through not changing things: this is why your Change Manager likes to say This is a UDR that does the work of calling the UPDATE STATISTICS commands. Auto Update Statistics Refresh cannot be called even manually without the database scheduler running.) in the ph_task table.

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What statistics and distributions should you maintain?

Informix uses a cost-based optimizer to determine how to run queries.

This relies on metadata to provide information about how large tables are, how many distinct values there are and other information about your data.

This risk is greatly reduced, if not eliminated, if the onconfig parameter OPTCOMPIND is set 0.

The downside of this is that the optimizer won’t select a sequential scan when it is the best query plan available unless there are no other options.

In general distributions on a column are more useful if there is some skew in the data.

However be aware that for non-indexed columns syscolumns.colmin and syscolumns.colmax are never populated by UPDATE STATISTICS LOW so the optimizer is truly blind about the data ranges without a distribution.Maintaining distributions on any sort of log or journal where there is a timestamp field can be a problem.The highest value in your distribution is wrong almost immediately after calculating it because new rows are being added all the time with later timestamps.The optimizer may prefer to drive off creation_date, particularly if the distribution indicates there are no data past the time the distribution was gathered.This is because it believes (wrongly) that selecting from a date range in the table. This is particularly true when there are a large number of accounts. Not really, this example is more to point out a danger.There may be also two filter conditions, one of which may very specific and pick out only a few rows; the other may be very general.

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