PostgreSQL 是什麼，跟 MySQL 有什麼差別

What's new in this version:

- Force the default wal_sync_method to be fdatasync on Linux The default on Linux has actually been fdatasync for many years, but recent kernel changes caused PostgreSQL to choose open_datasync instead. This choice did not result in any performance improvement, and caused outright failures on certain filesystems, notably ext4 with the data=journal mount option.
- Fix too many KnownAssignedXids error during Hot Standby replay 
- Fix race condition in lock acquisition during Hot Standby 
- Avoid unnecessary conflicts during Hot Standby This fixes some cases where replay was considered to conflict with standby queries (causing delay of replay or possibly cancellation of the queries), but there was no real conflict.
- Fix assorted bugs in WAL replay logic for GIN indexes This could result in bad buffer id: 0 failures or corruption of index contents during replication.
- Fix recovery from base backup when the starting checkpoint WAL record is not in the same WAL segment as its redo point
- Fix corner-case bug when streaming replication is enabled immediately after creating the master database cluster
- Fix persistent slowdown of autovacuum workers when multiple workers remain active for a long time The effective vacuum_cost_limit for an autovacuum worker could drop to nearly zero if it processed enough tables, causing it to run extremely slowly.
- Fix long-term memory leak in autovacuum launcher
- Avoid failure when trying to report an impending transaction wraparound condition from outside a transaction This oversight prevented recovery after transaction wraparound got too close, because database startup processing would fail.
- Add support for detecting register-stack overrun on IA64 The IA64 architecture has two hardware stacks. Full prevention of stack-overrun failures requires checking both.
- Add a check for stack overflow in copyObject() Certain code paths could crash due to stack overflow given a sufficiently complex query.
- Fix detection of page splits in temporary GiST indexes It is possible to have a concurrent page split in a temporary index, if for example there is an open cursor scanning the index when an insertion is done. GiST failed to detect this case and hence could deliver wrong results when execution of the cursor continued.
- Fix error checking during early connection processing The check for too many child processes was skipped in some cases, possibly leading to postmaster crash when attempting to add the new child process to fixed-size arrays.
- Improve efficiency of window functions Certain cases where a large number of tuples needed to be read in advance, but work_mem was large enough to allow them all to be held in memory, were unexpectedly slow. percent_rank(), cume_dist() and ntile() in particular were subject to this problem.
- Avoid memory leakage while ANALYZE'ing complex index expressions 
- Ensure an index that uses a whole-row Var still depends on its table An index declared like create index i on t (foo(t.-)) would not automatically get dropped when its table was dropped.
- Add missing support in DROP OWNED BY for removing foreign data wrapper/server privileges belonging to a user
- Do not inline a SQL function with multiple OUT parameters This avoids a possible crash due to loss of information about the expected result rowtype.
- Fix crash when inline-ing a set-returning function whose argument list contains a reference to an inline-able user function
- Behave correctly if ORDER BY, LIMIT, FOR UPDATE, or WITH is attached to the VALUES part of INSERT ... VALUES
- Make the OFF keyword unreserved (Heikki Linnakangas) This prevents problems with using off as a variable name in PL/pgSQL. That worked before 9.0, but was now broken because PL/pgSQL now treats all core reserved words as reserved.
- Fix constant-folding of COALESCE() expressions The planner would sometimes attempt to evaluate sub-expressions that in fact could never be reached, possibly leading to unexpected errors.
- Fix could not find pathkey item to sort planner failure with comparison of whole-row Vars
- Fix postmaster crash when connection acceptance (accept() or one of the calls made immediately after it) fails, and the postmaster was compiled with GSSAPI support
- Retry after receiving an invalid response packet from a RADIUS authentication server This fixes a low-risk potential denial of service condition.
- Fix missed unlink of temporary files when log_temp_files is active If an error occurred while attempting to emit the log message, the unlink was not done, resulting in accumulation of temp files.
- Add print functionality for InhRelation nodes This avoids a failure when debug_print_parse is enabled and certain types of query are executed.
- Fix incorrect calculation of distance from a point to a horizontal line segment This bug affected several different geometric distance-measurement operators.
- Fix incorrect calculation of transaction status in ecpg
- Fix errors in psql's Unicode-escape support
- Speed up parallel pg_restore when the archive contains many large objects (blobs)
- Fix PL/pgSQL's handling of simple expressions to not fail in recursion or error-recovery cases
- Fix PL/pgSQL's error reporting for no-such-column cases As of 9.0, it would sometimes report missing FROM-clause entry for table foo when record foo has no field bar would be more appropriate.
- Fix PL/Python to honor typmod (i.e., length or precision restrictions) when assigning to tuple fields This fixes a regression from 8.4.
- Fix PL/Python's handling of set-returning functions Attempts to call SPI functions within the iterator generating a set result would fail.
- Fix bug in contrib/cube's GiST picksplit algorithm This could result in considerable inefficiency, though not actually incorrect answers, in a GiST index on a cube column. If you have such an index, consider REINDEXing it after installing this update.
- Don't emit identifier will be truncated notices in contrib/dblink except when creating new connections
- Fix potential coredump on missing public key in contrib/pgcrypto
- Fix buffer overrun in contrib/pg_upgrade
- Fix memory leak in contrib/xml2's XPath query functions
- Update time zone data files to tzdata release 2010o for DST law changes in Fiji and Samoa; also historical corrections for Hong Kong.