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# Copyright 2003Curt Hill Hash indexes Are they better or worse than a B+Tree?

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Copyright 2003Curt Hill Hash indexes Are they better or worse than a B+Tree?

Copyright 2003Curt Hill Tasks Consider the basics of hashing Consider how this applies to indexing schemes Consider variations Consider the Hash Join

Copyright 2003Curt Hill Basics of hashing Internal hashing –A table in memory –Bucket usually holds one entry –Covered in a separate presentation: Hashing.ppt External hashing –On disk –Bucket is a page – holds multiple entries

Copyright 2003Curt Hill External Hashing Hash function takes the key and computes an integer How is this integer used? Direct file –Key is an integer Directory of heap file –Works well if one directory page can hold the correct number of page ids Lookup table –Converts integer to page id number

Copyright 2003Curt Hill How does it work? Hash function takes the key and computes a page number Search the page for the correct data page Access the data For very large indices the number of accesses can still be quite small

Copyright 2003Curt Hill Diagram Data... Hash buckets... 0 1 2 N-1

Copyright 2003Curt Hill Example Assume 1 million records with 4 records to a page –250000 pages of data Assume the key and pointer is 24 bytes with a 512 byte page –21 keys and pointers in a page Assume buckets are ¾ full –67000 buckets with 15 keys Hash function computes a value in range 0 – 66999 Without collisions it takes two accesses to get data

Copyright 2003Curt Hill Static and Dynamic Static hashing works well until the buckets fill up –Then a bucket requires an overflow bucket –Searching the original and overflow pages increases the accesses and performance drops Dynamic hashing involves techniques where the sizes may grow gracefully

Copyright 2003Curt Hill Extendible/Extensible Hashing Mechanism for altering the size of the hash table without the usual pain Common strategy for internal hashes is to double the hash table and rehash each entry This is too expensive for an index Instead we do incremental doubling of the buckets and index –Spreads the cost nicely

Copyright 2003Curt Hill Scheme We generate a hash that is in a range much larger than we need Typically modulo some large prime number Use only the bottom so many bits of that result to select the bucket Start the process with just one bit We also have the notion of global and local depth

Copyright 2003Curt Hill First example 0 1 1 1 21484 57911 1 Bit pattern ends in 0 Bit pattern ends in 1 Numbers shown are hash function output. Index Buckets

Copyright 2003Curt Hill Splitting a bucket The next insertion will overfill a bucket The exact action is dependent on the local and global levels If the local level = global level –Add one to global level (number of bits) –Double the index Add one more bit –Double the bucket Distribute values between the two If the local level < global level only double bucket

Copyright 2003Curt Hill Bucket and Index Split 0 1 1 1 21484 57911 1 2 00 01 10 11 1 2 2 24814 37 59 11 Add 3 – split 1 bucket into 01 and 11

Copyright 2003Curt Hill Continued Insertions Notice that there were two pointers to the unsplit bucket Insertions to a bucket that has a lower level than the global level only splits the bucket not the index It separates the two pointers

Copyright 2003Curt Hill Bucket Only Split 2 00 01 10 11 1 2 2 24814 59 7311 2 2 2 2 48 59 73 10 01 00 2 21014 Add 10 split bucket

Copyright 2003Curt Hill Extensible Hashing When the index exceeds one page –The upper so many bits may be checked so the entire index is not searched The mechanism is different than a tree The net effect is not that much different The index may grow smoothly without changes to the hash function or drastic rewriting

Copyright 2003Curt Hill Not without problems When the index is doubled there is work which is added to the insertion When the index will not fit in memory substantial I/Os occur When number of records per block is small we can end up with much larger global levels than needed –Suppose 2 records per block and 3 records have the same key for the last 20 bits –Global level of 20, even when most local levels are in 1-5 range

Copyright 2003Curt Hill Linear Hashing A different scheme with mechanism different than extensible hashing but some common properties –Splits are incrementally added –Some flexibility when they occur Like extensible hashing we use the bottom so many bits of a larger hash function Round robin bucket splitting Overflow buckets are used but may be later consumed

Copyright 2003Curt Hill Linear Hashing Numbers N – the number of buckets –Not always a power of two I – the number of used bits in the hash function R – the number of records in the structure M – the hash result –0  M  2 i –M may larger or smaller than N –If M > N we use M - 2 i-1

Copyright 2003Curt Hill Adding a bucket Any strategy can be used to determine when a bucket is added –Adding a bucket increases N When the ratio of records to buckets crosses a threshold When a bucket is forced to add an overflow bucket

Copyright 2003Curt Hill Linear Example I = 1 R = 3 N = 2 0000 1010 0 1 1111 R/N = 1.5 Split > 1.7

Copyright 2003Curt Hill Adding an Item When an item is added it is put in the proper bucket If if does not fit add an overflow bucket If the R/N threshold is crossed add a new bucket –This causes the corresponding bucket to be redistributed over the two buckets The number of hash bits used may be increased

Copyright 2003Curt Hill Insert 0101 I = 1 R = 3 N = 2 0000 1010 0 1 1111 Add 0101 to this Increases R/N ratio I = 2 R = 4 N = 3 0000 00 01 0101 1010 10 1111

Copyright 2003Curt Hill Explanation The bucket that was added to was not split –It was not its turn –Buckets are split in round robin fashion

Copyright 2003Curt Hill Insert 0111 I = 2 R = 4 N = 3 0000 00 01 0101 1010 10 1111 R/N = 1.3 I = 2 R = 5 N = 3 0000 00 01 0101 1010 10 1111 R/N = 1.67 0111 Add 0111 No split Overflow

Copyright 2003Curt Hill Insertion and Searching The hash function is now using two bits which gives it four possibilities, but there are only three buckets If the hash result M < N just use that bucket If the hash result M  N subtract from M 2 i-1 In last case 0111 was inserted –Last two bits are 11, but there is not yet a 11 bucket, so 10 was subtracted from it Searching uses same type of scheme

Copyright 2003Curt Hill Insert 1100 I = 2 R = 6 N = 4 0000 00 01 0101 1010 10 1111 0111 0000 00 01 0101 1010 10 1100 0111 11 1111 R/N = 1.5

Copyright 2003Curt Hill Dynamic Hashing Summary Linear hashing lacks the index of extensible hashing There are similarities –Hash function where only the bottom so many bits are used –Gradual splits –Quick lookups

Copyright 2003Curt Hill Joins If both files are sorted on the join the previously mentioned zipper join is used However, if the join field is not the primary key sorting the relation on this field may be expensive –Especially so if the outer join is larger than an inner join –The number of joined records is small compared to either relation size

Copyright 2003Curt Hill Hash Join Recall that a Cartesian Product makes all possible combinations of records from two relations –This could mean read numbering the products of block –That is exactly what we want to avoid Hash join partitions two relations into pieces based on a hash function Then only joins partitions that reacted similarly to the hash function Of course only works on Equi-Joins

Copyright 2003Curt Hill Hash Join Process Hash the smaller of the two files on the join field Read in the other file Hash each key into a bucket –The only candidates for equality are here Produce the output Smaller but still substantial

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