Обсуждение: Some indexing advice for a Postgres newbie, please?

Hi folks,

I have a single-user application which is growing beyond the
fixed-format data files in which it currently holds its data, I need a
proper database as the backend. The front end is written using Lazarus
and FreePascal under Linux, should anyone feel that makes a
difference. The database will need to grow to around 250,000 records.

My problem is with the data field which is the (unique) key. It's
really a single 192-bit integer (it holds various bits of bitmapped
data) which I currently hold as six 32-bit integers, but can convert
if needed when transferring the data.

How would you advise that I hold this field in a Postgres database,
given the requirement for the whole thing to be a unique key? The
first 64 bits change relatively infrequently, the last 128 bits will
change with virtually every record. The last 128 bits will ALMOST be
unique in themselves, but not quite. :(

Thanks,

Brian.

You should consider a BitString.
http://www.postgresql.org/docs/9.4/static/datatype-bit.html

On Thu, Feb 19, 2015 at 11:10 AM, brian <brian@meadows.pair.com> wrote:
>
> Hi folks,
>
> I have a single-user application which is growing beyond the
> fixed-format data files in which it currently holds its data, I need a
> proper database as the backend. The front end is written using Lazarus
> and FreePascal under Linux, should anyone feel that makes a
> difference. The database will need to grow to around 250,000 records.
>
> My problem is with the data field which is the (unique) key. It's
> really a single 192-bit integer (it holds various bits of bitmapped
> data) which I currently hold as six 32-bit integers, but can convert
> if needed when transferring the data.
>
> How would you advise that I hold this field in a Postgres database,
> given the requirement for the whole thing to be a unique key? The
> first 64 bits change relatively infrequently, the last 128 bits will
> change with virtually every record. The last 128 bits will ALMOST be
> unique in themselves, but not quite. :(
>
> Thanks,
>
> Brian.
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general

Hi folks, 

I have a single-user application which is growing beyond the
fixed-format data files in which it currently holds its data, I need a
proper database as the backend. The front end is written using Lazarus
and FreePascal under Linux, should anyone feel that makes a
difference. The database will need to grow to around 250,000 records. 

My problem is with the data field which is the (unique) key. It's
really a single 192-bit integer (it holds various bits of bitmapped
data) which I currently hold as six 32-bit integers, but can convert
if needed when transferring the data. 

How would you advise that I hold this field in a Postgres database,
given the requirement for the whole thing to be a unique key? The
first 64 bits change relatively infrequently, the last 128 bits will
change with virtually every record. The last 128 bits will ALMOST be
unique in themselves, but not quite. :( 

Thanks, 

Brian. 

On Thu, 19 Feb 2015 09:30:57 -0700, you wrote:

>On 02/19/2015 09:10 AM, brian wrote:
>> Hi folks,
>>
>> I have a single-user application which is growing beyond the
>> fixed-format data files in which it currently holds its data, I need a
>> proper database as the backend. The front end is written using Lazarus
>> and FreePascal under Linux, should anyone feel that makes a
>> difference. The database will need to grow to around 250,000 records.
>>
>> My problem is with the data field which is the (unique) key. It's
>> really a single 192-bit integer (it holds various bits of bitmapped
>> data) which I currently hold as six 32-bit integers, but can convert
>> if needed when transferring the data.
>>
>> How would you advise that I hold this field in a Postgres database,
>> given the requirement for the whole thing to be a unique key? The
>> first 64 bits change relatively infrequently, the last 128 bits will
>> change with virtually every record. The last 128 bits will ALMOST be
>> unique in themselves, but not quite. :(
>>
>> Thanks,
>>
>> Brian.
>>
>>
>If your application understands/parses/makes use of the data in those
>192 bites, I would reload with an additional unique id field. For the
>intended number of rows of data a sequence would be fine, though I'm
>partial to UUIDs. Alternatively map the 192 bytes to two fields and make
>a unique key of both of them. Third alternative would be to use a binary
>BitString a suggested by Brian.


Thanks. The purpose of the field is purely as a check against the user
feeding the same data in twice. Once I've constructed it, I never pull
the field apart again. It had to be done this way, as otherwise the
boolean statement to check for uniqueness was horrendous.

Brian.

On Thu, 19 Feb 2015 09:30:57 -0700, you wrote:

On 02/19/2015 09:10 AM, brian wrote:

Hi folks,

I have a single-user application which is growing beyond the
fixed-format data files in which it currently holds its data, I need a
proper database as the backend. The front end is written using Lazarus
and FreePascal under Linux, should anyone feel that makes a
difference. The database will need to grow to around 250,000 records.

My problem is with the data field which is the (unique) key. It's
really a single 192-bit integer (it holds various bits of bitmapped
data) which I currently hold as six 32-bit integers, but can convert
if needed when transferring the data.

How would you advise that I hold this field in a Postgres database,
given the requirement for the whole thing to be a unique key? The
first 64 bits change relatively infrequently, the last 128 bits will
change with virtually every record. The last 128 bits will ALMOST be
unique in themselves, but not quite. :(

Thanks,

Brian.

If your application understands/parses/makes use of the data in those 
192 bites, I would reload with an additional unique id field. For the 
intended number of rows of data a sequence would be fine, though I'm 
partial to UUIDs. Alternatively map the 192 bytes to two fields and make 
a unique key of both of them. Third alternative would be to use a binary 
BitString a suggested by Brian.

Thanks. The purpose of the field is purely as a check against the user
feeding the same data in twice. Once I've constructed it, I never pull
the field apart again. It had to be done this way, as otherwise the
boolean statement to check for uniqueness was horrendous. 

Brian. 

Hi,

On 19 Feb 2015 17:12, "brian" <brian@meadows.pair.com> wrote:
>
>
> Hi folks,
>
> I have a single-user application which is growing beyond the
> fixed-format data files in which it currently holds its data, I need a
> proper database as the backend. The front end is written using Lazarus
> and FreePascal under Linux, should anyone feel that makes a
> difference. The database will need to grow to around 250,000 records.
>
> My problem is with the data field which is the (unique) key. It's
> really a single 192-bit integer (it holds various bits of bitmapped
> data) which I currently hold as six 32-bit integers, but can convert
> if needed when transferring the data.
>
> How would you advise that I hold this field in a Postgres database,
> given the requirement for the whole thing to be a unique key? The
> first 64 bits change relatively infrequently, the last 128 bits will
> change with virtually every record. The last 128 bits will ALMOST be
> unique in themselves, but not quite. :(
>
> Thanks,
>
> Brian.
>

Postgres can use almost anything as a key so it probably depends on the library you use to access the database.

If it supports "composite primary keys" you can use the 6 ints as a key:

    PRIMARY KEY (n1, n2, n3, n4, n5, n6)

The numeric type can hold 192-bit numbers. I think Lazarus supports this as well.

You could also use a surrogate key and define a UNIQUE constraint on the 6 ints or the 192-bit number.

On Sat, Feb 21, 2015 at 5:33 AM, Arjen Nienhuis <a.g.nienhuis@gmail.com> wrote:
> Hi,
>
> On 19 Feb 2015 17:12, "brian" <brian@meadows.pair.com> wrote:
>>
>>
>> Hi folks,
>>
>> I have a single-user application which is growing beyond the
>> fixed-format data files in which it currently holds its data, I need a
>> proper database as the backend. The front end is written using Lazarus
>> and FreePascal under Linux, should anyone feel that makes a
>> difference. The database will need to grow to around 250,000 records.
>>
>> My problem is with the data field which is the (unique) key. It's
>> really a single 192-bit integer (it holds various bits of bitmapped
>> data) which I currently hold as six 32-bit integers, but can convert
>> if needed when transferring the data.
>>
>> How would you advise that I hold this field in a Postgres database,
>> given the requirement for the whole thing to be a unique key? The
>> first 64 bits change relatively infrequently, the last 128 bits will
>> change with virtually every record. The last 128 bits will ALMOST be
>> unique in themselves, but not quite. :(
>>
>> Thanks,
>>
>> Brian.
>>
>
> Postgres can use almost anything as a key so it probably depends on the
> library you use to access the database.
>
> If it supports "composite primary keys" you can use the 6 ints as a key:
>
>     PRIMARY KEY (n1, n2, n3, n4, n5, n6)
>
> The numeric type can hold 192-bit numbers. I think Lazarus supports this as
> well.
>
> You could also use a surrogate key and define a UNIQUE constraint on the 6
> ints or the 192-bit number.

You could also use 3 64 bit bigints if that's easier.  The other way
to do it is bytea.

merlin

On 25/02/15 15:42, Merlin Moncure wrote:
> On Sat, Feb 21, 2015 at 5:33 AM, Arjen Nienhuis <a.g.nienhuis@gmail.com> wrote:
>> Hi,
>>
>> On 19 Feb 2015 17:12, "brian" <brian@meadows.pair.com> wrote:
>>>
>>>
>>> Hi folks,
>>>
>>> I have a single-user application which is growing beyond the
>>> fixed-format data files in which it currently holds its data, I need a
>>> proper database as the backend. The front end is written using Lazarus
>>> and FreePascal under Linux, should anyone feel that makes a
>>> difference. The database will need to grow to around 250,000 records.
>>>
>>> My problem is with the data field which is the (unique) key. It's
>>> really a single 192-bit integer (it holds various bits of bitmapped
>>> data) which I currently hold as six 32-bit integers, but can convert
>>> if needed when transferring the data.
>>>
>>> How would you advise that I hold this field in a Postgres database,
>>> given the requirement for the whole thing to be a unique key? The
>>> first 64 bits change relatively infrequently, the last 128 bits will
>>> change with virtually every record. The last 128 bits will ALMOST be
>>> unique in themselves, but not quite. :(
>>>
>>> Thanks,
>>>
>>> Brian.
>>>
>>
>> Postgres can use almost anything as a key so it probably depends on the
>> library you use to access the database.
>>
>> If it supports "composite primary keys" you can use the 6 ints as a key:
>>
>>     PRIMARY KEY (n1, n2, n3, n4, n5, n6)
>>
>> The numeric type can hold 192-bit numbers. I think Lazarus supports this as
>> well.
>>
>> You could also use a surrogate key and define a UNIQUE constraint on the 6
>> ints or the 192-bit number.
>
> You could also use 3 64 bit bigints if that's easier.  The other way
> to do it is bytea.

The other way to do it is to have semantically-meaningful columns
rather than glomming them into this 192-bit integer, and a composite
key on the lot - if the set truly is unique.
--
Jeremy