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feat: Prefix Bloom Filter #103
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,11 @@ | ||
/// A trait allowing to customize prefix extraction in operations with bloom filter. | ||
/// It defines how prefix should be extracted from the key, when update or read | ||
/// a bloom filter. | ||
pub trait PrefixExtractor: Sync + Send { | ||
/// Extracts prefix from original key | ||
fn transform<'a>(&self, key: &'a [u8]) -> &'a [u8]; | ||
/// Checks if a key is in domain and prefix can be extracted from it. | ||
/// For example if `PrefixExtractor` suppose to extract first 4 bytes from key, | ||
/// `in_domain(&[0, 2, 3])` should return false | ||
fn in_domain(&self, key: &[u8]) -> bool; | ||
} |
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I don't think this is necessary.
If a key is [1, 2, 3] and we extract the first 4 bytes, we would just take the entire string, so:
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Let me better explain why is this needed in more practical example:
Imagine we have a database where keys are constructed using the following template:
{datatype}#{id}
and a prefix extractor that would extract{datatype}#
prefix from a key to optimize scans of all rows ofdatatype#
. Then, in table were populated following rowstable1#a
,table1#b
,table2#a
. The keys for bloom filter would betable1#
,table2#
in this case. Then, if we use prefix scan by prefixestable1#
ortable2#
everything will work correctly. But if scan is performed withtable
prefix withoutin_domain
the full prefix will be extracted and will try to check bloom filter bytable
key, which may return false negative.in_domain
in this case will help to prevent such cases and will use Bloom Filter only in case when prefix is in domain. In this example everything starting with{datatype}#
will be considered asin_domain
and use Bloom Filter, other prefixes will benot in_domain
and will just use normal prefix search ignoring Bloom FilterThere was a problem hiding this comment.
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Makes sense.
However, for the prefix extractor to support structured prefixes, the trait needs to return an Iterator over extracted prefixed (see #97 and fjall-rs/fjall#116). At that point, the trait could just return
std::iter::empty
, which would signal that the key is "not in domain".For example we may extract prefixes, like:
eu#germany#berlin
-> extract "eu#" as prefix
-> also extract "eu#germany#"
Now prefix searches over both continent and [continent + country] can be filtered by the prefix filter.