Index Locks in Java

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Index Locks
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You can lock B-tree index pages the same way that you lock table pages, but beware: index pages are "shiftier" (because data-change statements cause shifting and splitting), "big-grainier" (because typical index pages hold many small keys), and "bottleneckier" (because all index searches begin with the same page at the top node) Some special tweaks are therefore necessary We'll tell you about them, with the warning that these considerations do not apply in all cases 1 The DBMS will release index locks early During a SELECT via an index, shared locks are set for the index and for the table pages that the matching index keys point to In READ COMMITTED transactions, the index locks disappear after the SELECT is executed instead of after the fetch, as the following example shows
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SELECT /* index locks released here */ FETCH /* table locks released here */
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7 The DBMS will split upper-level index pages before they're full Suppose the current index has a nearly full top node pointing to a nearly empty leaf node In this situation, the DBMS cannot travel all the way down, split a leaf node, then travel all the way back splitting upper nodes as it goes that would cause deadlocks So the DBMS can, and will, split top nodes "in anticipation," even when such action is unnecessary 8 The DBMS will make an effort to prevent keys from shifting during a transaction For example, Microsoft and Oracle will only mark a deleted key as "deleted" rather than actually removing the key and shifting all following keys backward As another example, Sybase will add a pointer to an overflow page rather than actually inserting a new key and shifting all following keys forward 9 The DBMS will use marks on the wall to stop concurrent transactions from passing through the same place Consider the situation shown in Table 15-10, where Transaction #1 has placed a mark on the wall for two index keys Table 15-10 Marks on the Wall in an Index Index Leaf Page BELGRADE LJUBLJANA SARAJEVO SKOPJE ZAGREB
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Transaction #1 puts a mark on the wall here Transaction #1 puts a mark on the wall here
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10 Now, if Transaction #2 does this SELECT:
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11 SELECT * FROM Table1 12 WHERE column1 BETWEEN 'BELGRADE' AND 'SARAJEVO' 13 it will encounter Transaction #1's mark on LJUBLJANA and wait for it to be removed Meanwhile, if Transaction #1 actually deletes LJUBLJANA, the mark disappears, which is
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another reason that index keys can't be physically deleted during a transaction This "key range locking" is an effective way to lock index keys instead of index pages The flaw is the horrific amount of time that it takes Key range locks occur only in transactions with isolation level SERIALIZABLE
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14 The DBMS will treat a lock for a strong-clustered table as if it's an index lock, not a table lock That has to happen because a clustered table is organized as a B-tree This constitutes a warning that the locking rules change when the table is clustered As for bitmap indexes, they don't present special locking problems Updating bitmap indexes is slow because each row is indexed in multiple pages; however, the bitmap is in ROWID order so the B-tree's difficulties (shifts, big-grain locks, and bottlenecks) won't come up That's not the case with SERIALIZABLE transactions, which are simply incompatible with bitmap indexes
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The Bottom Line: Index Locks
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Index locks favor READ COMMITTED transactions Changes to index keys cause unexpected splits and blocks Putting one index on a table doesn't merely double the locking: effects are geometric
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"Nobody goes there anymore because it's too crowded" Yogi Berra When we say Moose Jaw is the hot spot to be this year, we're talking not about weather but about popularity Similarly, a hot spot isn't exactly where the disk drive head is burning from friction Rather, it's an object that many processes are trying to get at the same time Not all hot spots are hellish If every transaction is READ UNCOMMITTED, or if traffic is light, then it's wonderful to find that a page is still in the cache, left over from the last user's access The hot spots we're talking about here, though, are the malignant kind: hot spots that cause exclusive locks and block other processes INSERT statements cause hot spots because the DBMS tries to put all new rows at the end of the table file-section (unless it's a clustered table) Because the table file has only one end, INSERT statements always try to get locks for the last page in the file The DBMS can help here; for example, Microsoft can use row locks rather than page locks for INSERTs Another palliative is to use clustered indexes (recall from 9, "Indexes," that, with clustered indexes, newly inserted rows can be anywhere in the file because they're put in order by the cluster key) Index splits cause hot spots too, because DBMSs try to put new index pages together in the index file The DBA can avoid such hot spots by setting some parameters to make the DBMS look first at "the end of the extent" rather than "the end of the file," although that technique wastes space Globals also cause hot spots Typical examples are reporting applications that change a single row containing a "date of last report" field or bank withdrawals that immediately update the general ledger Another example of a global is an interest rate: if interest rates change only once a day, can you read them in once to a program variable instead of looking them up in a table Another cause of hot spots is using sequences, or monotonics If you use sequencing that is, if you use SERIAL data types or IDENTITY columns, or auto-incrementing objects here is a question for
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you: Given that the number following 12345678 is 12345679, what is the probability that two keys containing those values will end up on the same index page If you answered nearly 100%, you're right! There are three easy fixes for sequence hot spots
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Store the digits of the sequence in reverse; that is, store the numbers as 87654321 and 97654321 Because the difference is now at the start rather than at the end of the key, the keys won't sort near each other Oracle supports this idea explicitly with its "reverse indexing" option Of course, it's not such a good idea if you often ORDER BY sequence_column when you SELECT Use a unique-key generator rather than a true sequence For example, a stored procedure can form a Microsoft Global Unique Identifier (GUID) using the network connection information Node ID, the date, and the time of day to a high precision Add the sequence number to some nearly unique value derived from the row itself, such as a surname If someone objects that sequence numbers are useful because they can easily tell how many rows there are by looking at MAX(sequence_column), we counter that rows can be deleted, transactions can be rolled back after a sequence number is generated, and UNION can cause an increment, and all these things muck up the nice monotonic sequence And what's more, global sequence numbers are a cause of locking trouble in themselves, because the DBMS has to put a latch on the number while incrementing and retrieving it
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