[DUG] Using Boolean (Char(1)) in Firebird

[Jolyon Smith]

I don't think you can adopt a general rule for all boolean type conditions
in data.  In the two example fields you cite, for example, I can see that
there is a potential difference in the nature of the booleans involved.

ActiveRecord - looks like something that could change over time.  A record
that was active may become inactive and I further speculate that there will
over time be far more inactive records than active ones.

AccountTransactionType - looks like something that is fixed.  The type of a
transaction seems unlikely to change once that transaction has been
recorded.  You might call this a "static" boolean, as opposed to the more
"dynamic" nature of the previous example.


Of course, more specific domain knowledge may reveal these assumptions to
be invalid, but you get the general idea.... the characteristics of a
particular datum go beyond it's simple data type and those characteristics
in turn determine the most appropriate implementation (which in turn will
depend on whether the dominant context is OLTP or OLAP - i.e. efficiency of
creating/modifying data vs efficiency of queries).


In the case of "static" booleans for example, you might consider creating
separate tables for records of different values in this field.  For
convenience of querying all records you can of course project a view which
unions the two (or more) tables involved, with a derived, virtual column
containing the discriminating field value.  This also opens up the
possibility that the most efficient indexes for rows of a certain type
(i.e. now table) may well be different than those for the other.  i.e. the
way you work with Income transactions might benefit from different indexes
than Expense transactions.

On the other hand, the way you work with income and expense transactions
may mean that you are better off having indexes operating over ALL
transactions, regardless of Income/Expense type.

See what I mean about "the best way" being dependent on far more than just
the data type ?

And there's still more to it than that...

w.r.t index selectivity, I am not convinced that the 1 / # of distinct
values metric is a particularly reliable measure.  It surely assumes an
even distribution of distinct values across the data set ?

i.e. if you have 100,000 records and they have a column where 50,000 rows
have one value and 50,000 have another, then yes, the efficiency and thus
the utility of any index on that value is going to be negligible (but then,
no better than having no index isn't actually *worse*, is it ?  Although
there will be some overhead introduced in maintaining the index, though I
doubt this will itself be hugely significant).

On the other hand, if only 1,000 of those 100,000 records have one value
and the remaining 99,000 have another, AND if your application most often
queries that table to select those in the smaller subset (the 1,000) then
whilst an index may not be of any benefit for querying the 99,000, it
surely will provide benefit for those queries that select the 1,000 (or
from among them), a benefit which *might* be worth the overhead of
maintaining that index even though it provides little/no benefit for the
handful/minority of queries that work with the 99,000 records ?


The bottom line is, there is no shortcut for properly understanding your
data and the way your application(s) work(s) with that data for correctly
tuning your database structure and metadata for optimal performance.

:)


On 30 March 2014 15:19, Steve Peacocke <steve at peacocke.net> wrote:

> Hi all,
>
> I'm playing around with a Firebird database and wanted to know from you DB
> experts out there how you handle booleans in a table.
>
> These could be as simple as
>   ActiveRecord (Y/N)
>   AccountTransactionType (I/E) - (Income or Expense)
>
> That last I would normally think would be "Income (Y/N)" so that would be
> a boolean too.
>
> My understanding is that this will never be indexed, even if you
> specifically add an index to it. So how do you handle it. There may be
> several boolean fields in a table definition.
>
> As these tables c an contain several hundred thousand records, this could
> potentially slow down any query to say total all records last 3 years where
> Active and Income - as the only index would then be on the date field,
> there is a possibility that this could potentially be a very slow query.
>
> I've heard of others creating another table to create, say, non-Avtive
> record ID's, but this one table could have several booleans, therefore
> creating several new tables (combining then into a single table with the
> field name would cause the same problem).
>
> Any thoughts?
>
> Steve Peacocke
> Mobile: +64 220 612-611
> Linkedin Professional Profile<http://nz.linkedin.com/pub/steve-peacocke/1/a06/489>
>
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