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

Wed Apr 2 22:14:24 NZDT 2014

Rodney,

There are some shortcomings to this approach. Firebird does support "function based indexes" .. but would require a separate index for every combination that you want to search since that "5" in the "BIN_AND(CombinedBoolean, 5)" function call has to be static to create the index. So If you use that field to store something like 8 flags then you are looking at a possible 256 different indexes.

As far as I know there is no perfect solution to a problem like this ... b+tree indexes (as used in most relational databases) and low cardinality of data just don't mix well performance wise. Unless your databases supports bitmap-indexes (oracle does) there isn't much you can do other than invest in SSD/PCIe drives/memory to make up for the slow full table scans that will be required.

Regards,
Stefan

-----Original Message-----
From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Rodney
Sent: Wednesday, 2 April 2014 11:02 a.m.
To: delphi at listserver.123.net.nz
Subject: Re: [DUG] Using Boolean (Char(1)) in Firebird

How about adding another field as a combination of Boolean fields?

For example:

Add a field called CombinedBoolean as Integer, each bit of this field represent one Boolean field, like: IsActive (bit2), IsCustomer (bit1), IsLocal (bit0)

Create a before-post trigger to update this field on the server.

Create an index on CombinedBoolean.

When doing the query, construct the query value according to the need.  Say 'Select .... Where BIN_AND(CombinedBoolean, 5) = 5' means Active and Local.

Remark:
1.  I don't have a FB machine to try but the BIN_AND() function should be available since FB 2.1 2.  I am not sure whether it will improve the performance or not.  Just try to post an idea for further discussion.  Please let me know if it is wrong.

Cheers
Rodney C.

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Today's Topics:

   1. Re: Using Boolean (Char(1)) in Firebird (Stefan Mueller)
   2. What is your experience with oxygene? (Leigh Wanstead)

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Message: 1
Date: Mon, 31 Mar 2014 13:45:27 +1300
From: "Stefan Mueller" <muellers at orcl-toolbox.com>
Subject: Re: [DUG] Using Boolean (Char(1)) in Firebird
To: "'NZ Borland Developers Group - Delphi List'"
	<delphi at listserver.123.net.nz>
Message-ID: <002201cf4c7a$836eee00$8a4cca00$@orcl-toolbox.com>
Content-Type: text/plain; charset="utf-8"

Jolyon already mentioned a lot of good things to look at (how often you update the data, the cardinality/distribution of the data you index, etc)

I am more of an Oracle guy and never had any experience with FireBird/InterBase ? but I guess those two characteristics will apply to any RDBMS system:

1.) Maintaining indexes takes time. Any update/insert/delete on the data means indexes also have to be touched. So make sure you get the balance between write/read right and not overindex things. 

2.) scanning indexes and then looking up the datarow for it costs time too.
For Oracle the golden rule of thumb is to forget about indexes if you access more than 15% of the table - a full table scan will be faster than an index.
That?s just a guide, hardcore performance tuning specialists look at length of data stored in a row and storage block size and a few other things and depending on those numbers it can be as low as 5% or as high as 30% (for tables with very small rows). I am assuming that Firebird probably isn?t doing as much optimization for caching data as Oracle does, in which case those %-numbers might even be lower for Firebird than with Oracle.

Kind regards,

Stefan M?ller,
R&D Manager

ORCL Toolbox Ltd. 
Auckland, New Zealand 

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From: delphi-bounces at listserver.123.net.nz
[mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Jolyon Smith
Sent: Monday, 31 March 2014 8:56 a.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] Using Boolean (Char(1)) in Firebird

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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Message: 2
Date: Tue, 1 Apr 2014 11:30:50 +1300
From: Leigh Wanstead <leigh.wanstead at gmail.com>
Subject: [DUG] What is your experience with oxygene?
To: NZ Borland Developers Group - Delphi List <delphi at delphi.org.nz>
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	<CAAAyFX2WL9UvYvb5GsOWt+Q0Wk2ENTPWtCPGLL+FyUXrzpwq3A at mail.gmail.com>
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Good morning,

What is your experience with oxygene? How is it compare to Delphi Xe5 with mobile device for android and ios?

http://www.remobjects.com/elements/oxygene/

TIA

Regards
Leigh
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