16.02.2022 11:19, Mark Rotteveel wrote:
On 2022-02-16 09:56, Vlad Khorsun wrote:
16.02.2022 10:35, Mark Rotteveel wrote:
On 2022-02-15 20:08, Vlad Khorsun wrote:
15.02.2022 20:32, Mark Rotteveel wrote:
On 2022-02-15 16:20, Vlad Khorsun wrote:
I'd vote to remove idx_numeric2 in favour of idx_decimal and look
how to improve
Decimal128::makeIndexKey() performance. For example, it stores every 3 decimal
digits into 10 bits using relatively complex (computationally) algorithm with
shifts and multiplications. Storing every digit in 4 bits will use 2 bits per 3
digits more but should save come CPU circles, I believe.
But that is the storage format of Decimal128 (DECFLOAT(34)).
No. It looks more like packed BCD. DECFLOAT uses a more complex internals,
it is not BCD inside.
Decimal128 uses densely packed BCD (with some oddities to encode sign, exponent
and first digit).
It is not BCD and definitely not what uses Decimal128::makeIndexKey().
See http://speleotrove.com/decimal/dbspec.html
and http://speleotrove.com/decimal/DPDecimal.html
Also, note, Decimal encodings is not directly sortable.
From that first link under 'coefficient continuation': "Each 10-bit group represents three decimal digits, using Densely Packed
Decimal encoding."
Sure. And it is not BCD, nor packed BCD ;)
and at the footnote (and the top of your second link): "Chen-Ho encoding is a lossless compression of three
Binary Coded Decimal digits into 10 bits using an algorithm which can be applied or reversed using only simple Boolean operations".
In any case - encoding used by Decimal128::makeIndexKey() is not the
same encoding as used by Decimal internally.
However, that was not my main point. My main point was that it sounds like an index format that was created for supporting
DECFLOAT(34), and that it is not suitable for the full range of INT128 and NUMERIC/DECIMAL backed by INT128 (for the same reasons
the DOUBLE PRECISION format is not suitable for BIGINT and NUMERIC/DECIMAL backed by BIGINT).
And I never object it. Note, I offer to remove idx_numeric2 (used for INT64).
Regards,
Vlad
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