@@ -51,13 +51,13 @@ if a transaction can be shown to always do the right thing when it is
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run alone (before or after any other transaction), it will always do
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the right thing in any mix of concurrent serializable transactions.
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Where conflicts with other transactions would result in an
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- inconsistent state within the database, or an inconsistent view of
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+ inconsistent state within the database or an inconsistent view of
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the data, a serializable transaction will block or roll back to
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prevent the anomaly. The SQL standard provides a specific SQLSTATE
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for errors generated when a transaction rolls back for this reason,
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so that transactions can be retried automatically.
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- Before version 9.1 PostgreSQL did not support a full serializable
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+ Before version 9.1, PostgreSQL did not support a full serializable
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isolation level. A request for serializable transaction isolation
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actually provided snapshot isolation. This has well known anomalies
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which can allow data corruption or inconsistent views of the data
@@ -77,7 +77,7 @@ Serializable Isolation Implementation Strategies
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Techniques for implementing full serializable isolation have been
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published and in use in many database products for decades. The
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- primary technique which has been used is Strict2 Phase Locking
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+ primary technique which has been used is StrictTwo- Phase Locking
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(S2PL), which operates by blocking writes against data which has been
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read by concurrent transactions and blocking any access (read or
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write) against data which has been written by concurrent
@@ -112,54 +112,90 @@ visualize the difference between the serializable implementations
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described above, is to consider that among transactions executing at
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the serializable transaction isolation level, the results are
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required to be consistent with some serial (one-at-a-time) execution
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- of the transactions[1]. How is that order determined in each?
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+ of the transactions [1]. How is that order determined in each?
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- S2PL locks rows used by the transaction in a way which blocks
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- conflicting access, so that at the moment of a successful commit it
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- is certain that no conflicting access has occurred. Some transactions
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- may have blocked, essentially being partially serialized with the
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- committing transaction, to allow this. Some transactions may have
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- been rolled back, due to cycles in the blocking. But with S2PL,
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- transactions can always be viewed as having occurred serially, in the
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- order of successful commit.
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+ In S2PL, each transaction locks any data it accesses. It holds the
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+ locks until committing, preventing other transactions from making
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+ conflicting accesses to the same data in the interim. Some
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+ transactions may have to be rolled back to prevent deadlock. But
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+ successful transactions can always be viewed as having occurred
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+ sequentially, in the order they committed.
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With snapshot isolation, reads never block writes, nor vice versa, so
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- there is much less actual serialization. The order in which
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- transactions appear to have executed is determined by something more
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- subtle than in S2PL: read/write dependencies. If a transaction
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- attempts to read data which is not visible to it because the
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- transaction which wrote it (or will later write it) is concurrent
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- (one of them was running when the other acquired its snapshot), then
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- the reading transaction appears to have executed first, regardless of
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- the actual sequence of transaction starts or commits (since it sees a
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- database state prior to that in which the other transaction leaves
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- it). If one transaction has both rw-dependencies in (meaning that a
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- concurrent transaction attempts to read data it writes) and out
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- (meaning it attempts to read data a concurrent transaction writes),
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- and a couple other conditions are met, there can appear to be a cycle
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- in execution order of the transactions. This is when the anomalies
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- occur.
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-
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- SSI works by watching for the conditions mentioned above, and rolling
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- back a transaction when needed to prevent any anomaly. The apparent
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- order of execution will always be consistent with any actual
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- serialization (i.e., a transaction which run by itself can always be
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- considered to have run after any transactions committed before it
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- started and before any transacton which starts after it commits); but
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- among concurrent transactions it will appear that the transaction on
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- the read side of a rw-dependency executed before the transaction on
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- the write side.
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+ more concurrency is possible. The order in which transactions appear
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+ to have executed is determined by something more subtle than in S2PL:
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+ read/write dependencies. If a transaction reads data, it appears to
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+ execute after the transaction that wrote the data it is reading.
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+ Similarly, if it updates data, it appears to execute after the
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+ transaction that wrote the previous version. These dependencies, which
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+ we call "wr-dependencies" and "ww-dependencies", are consistent with
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+ the commit order, because the first transaction must have committed
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+ before the second starts. However, there can also be dependencies
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+ between two *concurrent* transactions, i.e. where one was running when
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+ the other acquired its snapshot. These "rw-conflicts" occur when one
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+ transaction attempts to read data which is not visible to it because
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+ the transaction which wrote it (or will later write it) is
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+ concurrent. The reading transaction appears to have executed first,
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+ regardless of the actual sequence of transaction starts or commits,
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+ because it sees a database state prior to that in which the other
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+ transaction leaves it.
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+
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+ Anomalies occur when a cycle is created in the graph of dependencies:
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+ when a dependency or series of dependencies causes transaction A to
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+ appear to have executed before transaction B, but another series of
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+ dependencies causes B to appear before A. If that's the case, then
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+ the results can't be consistent with any serial execution of the
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+ transactions.
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+
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+
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+ SSI Algorithm
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+ -------------
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+
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+ Serializable transaction in PostgreSQL are implemented using
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+ Serializable Snapshot Isolation (SSI), based on the work of Cahill
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+ et al. Fundamentally, this allows snapshot isolation to run as it
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+ has, while monitoring for conditions which could create a serialization
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+ anomaly.
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+
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+ SSI is based on the observation [2] that each snapshot isolation
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+ anomaly corresponds to a cycle that contains a "dangerous structure"
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+ of two adjacent rw-conflict edges:
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+
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+ Tin ------> Tpivot ------> Tout
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+ rw rw
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+
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+ SSI works by watching for this dangerous structure, and rolling
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+ back a transaction when needed to prevent any anomaly. This means it
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+ only needs to track rw-conflicts between concurrent transactions, not
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+ wr- and ww-dependencies. It also means there is a risk of false
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+ positives, because not every dangerous structure corresponds to an
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+ actual serialization failure.
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+
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+ The PostgreSQL implementation uses two additional optimizations:
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+
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+ * Tout must commit before any other transaction in the cycle
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+ (see proof of Theorem 2.1 of [2]). We only roll back a transaction
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+ if Tout commits before Tpivot and Tin.
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+
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+ * if Tin is read-only, there can only be an anomaly if Tout committed
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+ before Tin takes its snapshot. This optimization is an original
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+ one. Proof:
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+
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+ - Because there is a cycle, there must be some transaction T0 that
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+ precedes Tin in the serial order. (T0 might be the same as Tout).
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+
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+ - The dependency between T0 and Tin can't be a rw-conflict,
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+ because Tin was read-only, so it must be a wr-dependency.
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+ Those can only occur if T0 committed before Tin started.
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+
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+ - Because Tout must commit before any other transaction in the
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+ cycle, it must commit before T0 commits -- and thus before Tin
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+ starts.
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PostgreSQL Implementation
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-------------------------
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- The implementation of serializable transactions for PostgreSQL is
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- accomplished through Serializable Snapshot Isolation (SSI), based on
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- the work of Cahill, et al. Fundamentally, this allows snapshot
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- isolation to run as it has, while monitoring for conditions which
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- could create a serialization anomaly.
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-
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* Since this technique is based on Snapshot Isolation (SI), those
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areas in PostgreSQL which don't use SI can't be brought under SSI.
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This includes system tables, temporary tables, sequences, hint bit
@@ -180,7 +216,7 @@ lock or to use SELECT FOR SHARE or SELECT FOR UPDATE.
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* Those who want to continue to use snapshot isolation without
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the additional protections of SSI (and the associated costs of
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enforcing those protections), can use the REPEATABLE READ transaction
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- isolation level. This levelwill retain its legacy behavior, which
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+ isolation level. This levelretains its legacy behavior, which
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is identical to the old SERIALIZABLE implementation and fully
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consistent with the standard's requirements for the REPEATABLE READ
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transaction isolation level.
@@ -236,7 +272,7 @@ in PostgreSQL, but tailored to the needs of SIREAD predicate locking,
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are used. These refer to physical objects actually accessed in the
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course of executing the query, to model the predicates through
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inference. Anyone interested in this subject should review the
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- Hellerstein, Stonebraker and Hamilton paper[2 ], along with the
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+ Hellerstein, Stonebraker and Hamilton paper [3 ], along with the
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locking papers referenced from that and the Cahill papers.
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Because the SIREAD locks don't block, traditional locking techniques
@@ -273,6 +309,15 @@ transaction already holds a write lock on any tuple representing the
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row, since a rw-dependency would also create a ww-dependency which
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has more aggressive enforcement and will thus prevent any anomaly.
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+ * Modifying a heap tuple creates a rw-conflict with any transaction
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+ that holds a SIREAD lock on that tuple, or on the page or relation
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+ that contains it.
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+
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+ * Inserting a new tuple creates a rw-conflict with any transaction
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+ holding a SIREAD lock on the entire relation. It doesn't conflict with
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+ page-level locks, because page-level locks are only used to aggregate
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+ tuple locks. Unlike index page locks, they don't lock "gaps" on the page.
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+
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Index AM implementations
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------------------------
@@ -296,13 +341,13 @@ need not generate a conflict, although an update which "moves" a row
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into the scan must generate a conflict. While correctness allows
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false positives, they should be minimized for performance reasons.
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- Several optimizations are possible:
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+ Several optimizations are possible, though not all implemented yet :
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* An index scan which is just finding the right position for an
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- index insertion or deletionneed not acquire a predicate lock.
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+ index insertion or deletionneeds not acquire a predicate lock.
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* An index scan which is comparing for equality on the entire key
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- for a unique indexneed not acquire a predicate lock as long as a key
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+ for a unique indexneeds not acquire a predicate lock as long as a key
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is found corresponding to a visible tuple which has not been modified
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by another transaction -- there are no "between or around" gaps to
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cover.
@@ -317,10 +362,10 @@ x = 1 AND x = 2), then no predicate lock is needed.
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Other index AM implementation considerations:
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- *If a btree search discovers that no root page has yet been
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- created, a predicate lock on the indexrelation is required;
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- otherwise btree searches must get to the leaf level to determine
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- which tuples match, so predicate locks go there .
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+ *B-tree index searches acquire predicate locks only on the
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+ index *leaf* pages needed to lock theappropriate indexrange. If,
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+ however, a search discovers that no root page has yet been created, a
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+ predicate lock on the index relation is required .
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* GiST searches can determine that there are no matches at any
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level of the index, so there must be a predicate lock at each index
@@ -346,11 +391,6 @@ to be added from scratch.
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2. The existing in-memory lock structures were not suitable for
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tracking SIREAD locks.
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- * The database products used for the prototype
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- implementations for the papers used update-in-place with a rollback
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- log for their MVCC implementations, while PostgreSQL leaves the old
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- version of a row in place and adds a new tuple to represent the row
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- at a new location.
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* In PostgreSQL, tuple level locks are not held in RAM for
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any length of time; lock information is written to the tuples
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involved in the transactions.
@@ -450,18 +490,19 @@ there can't be a rw-conflict from T3 to T0.
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o In both cases, we didn't need the T1 -> T3 edge.
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- * Predicate locking in PostgreSQLwill start at the tuple level
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- when possible, with automatic conversion of multiple fine-grained
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- locks to coarser granularity asneed to avoid resource exhaustion.
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- The amount of memory used for these structureswill be configurable,
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- to balance RAM usage against SIREAD lock granularity.
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+ * Predicate locking in PostgreSQLstarts at the tuple level
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+ when possible. Multiple fine-grained locks are promoted to a single
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+ coarser- granularitylock asneeded to avoid resource exhaustion. The
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+ amount of memory used for these structuresis configurable, to balance
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+ RAM usage against SIREAD lock granularity.
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- * A process-local copy of locks held by a process and the coarser
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- covering locks with counts, are kept to support granularity promotion
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- decisions with low CPU and locking overhead.
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+ * Each backend keeps a process-local table of the locks it holds.
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+ To support granularity promotion decisions with low CPU and locking
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+ overhead, this table also includes the coarser covering locks and the
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+ number of finer-granularity locks they cover.
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- * Conflictswill be identified by looking for predicate locks
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- when tuples are written and looking at the MVCC information when
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+ * Conflictsare identified by looking for predicate locks
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+ when tuples are written, and by looking at the MVCC information when
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tuples are read. There is no matching between two RAM-based locks.
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* Because write locks are stored in the heap tuples rather than a
@@ -493,12 +534,12 @@ to be READ ONLY.)
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o We can more aggressively clean up conflicts, predicate
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locks, and SSI transaction information.
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- *Allow a READ ONLY transaction to "opt out" of SSI if there are
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+ *We allow a READ ONLY transaction to "opt out" of SSI if there are
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no READ WRITE transactions which could cause the READ ONLY
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transaction to ever become part of a "dangerous structure" of
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overlapping transaction dependencies.
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- *Allow the user to request that a READ ONLY transaction wait
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+ *We allow the user to request that a READ ONLY transaction wait
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until the conditions are right for it to start in the "opt out" state
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described above. We add a DEFERRABLE state to transactions, which is
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specified and maintained in a way similar to READ ONLY. It is
@@ -538,28 +579,13 @@ address it?
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replication solutions, like Postgres-R, Slony, pgpool, HS/SR, etc.
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This is related to the "WAL file replay" issue.
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- * Weak-memory-ordering machines. Make sure that shared memory
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- access which involves visibility across multiple transactions uses
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- locks as needed to avoid problems. On the other hand, ensure that we
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- really need volatile where we're using it.
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- http://archives.postgresql.org/pgsql-committers/2008-06/msg00228.php
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-
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* UNIQUE btree search for equality on all columns. Since a search
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of a UNIQUE index using equality tests on all columns will lock the
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heap tuple if an entry is found, it appears that there is no need to
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get a predicate lock on the index in that case. A predicate lock is
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still needed for such a search if a matching index entry which points
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to a visible tuple is not found.
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- * Planner index probes. To avoid problems with data skew at the
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- ends of an index which have historically caused bad plans, the
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- planner now probes the end of an index to see what the maximum or
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- minimum value is when a query appears to be requesting a range of
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- data outside what statistics shows is present. These planner checks
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- don't require predicate locking, but there's currently no easy way to
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- avoid it. What can we do to avoid predicate locking for such planner
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- activity?
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-
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* Minimize touching of shared memory. Should lists in shared
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memory push entries which have just been returned to the front of the
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available list, so they will be popped back off soon and some memory
@@ -573,13 +599,17 @@ Footnotes
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[1] http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
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Search for serial execution to find the relevant section.
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- [2] http://db.cs.berkeley.edu/papers/fntdb07-architecture.pdf
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- Joseph M. Hellerstein, Michael Stonebraker and James Hamilton. 2007.
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+ [2] A. Fekete et al. Making Snapshot Isolation Serializable. In ACM
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+ Transactions on Database Systems 30:2, Jun. 2005.
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+ http://dx.doi.org/10.1145/1071610.1071615
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+
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+ [3] Joseph M. Hellerstein, Michael Stonebraker and James Hamilton. 2007.
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Architecture of a Database System. Foundations and Trends(R) in
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Databases Vol. 1, No. 2 (2007) 141-259.
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+ http://db.cs.berkeley.edu/papers/fntdb07-architecture.pdf
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Of particular interest:
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* 6.1 A Note on ACID
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* 6.2 A Brief Review of Serializability
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* 6.3 Locking and Latching
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* 6.3.1 Transaction Isolation Levels
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- * 6.5.3 Next-Key Locking: Physical Surrogates for Logical
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+ * 6.5.3 Next-Key Locking: Physical Surrogates for Logical Properties