PostgreSQL 是什麼，跟 MySQL 有什麼差別

What's new in this version:

- Fix incorrect password transformation in contrib/pgcrypto's DES crypt() function (Solar Designer) If a password string contained the byte value 0x80, the remainder of the password was ignored, causing the password to be much weaker than it appeared. With this fix, the rest of the string is properly included in the DES hash. Any stored password values that are affected by this bug will thus no longer match, so the stored values may need to be updated. 
- Ignore SECURITY DEFINER and SET attributes for a procedural language's call handler Applying such attributes to a call handler could crash the server. 
- Make contrib/citext's upgrade script fix collations of citext arrays and domains over citext Release 9.1.2 provided a fix for collations of citext columns and indexes in databases upgraded or reloaded from pre-9.1 installations, but that fix was incomplete: it neglected to handle arrays and domains over citext. This release extends the module's upgrade script to handle these cases. As before, if you have already run the upgrade script, you'll need to run the collation update commands by hand instead. See the 9.1.2 release notes for more information about doing this.
- Allow numeric timezone offsets in timestamp input to be up to 16 hours away from UTC Some historical time zones have offsets larger than 15 hours, the previous limit. This could result in dumped data values being rejected during reload.
- Fix timestamp conversion to cope when the given time is exactly the last DST transition time for the current timezone This oversight has been there a long time, but was not noticed previously because most DST-using zones are presumed to have an indefinite sequence of future DST transitions.
- Fix text to name and char to name casts to perform string truncation correctly in multibyte encodings 
- Fix memory copying bug in to_tsquery() 
- Ensure txid_current() reports the correct epoch when executed in hot standby 
- Fix planner's handling of outer PlaceHolderVars within subqueries This bug concerns sub-SELECTs that reference variables coming from the nullable side of an outer join of the surrounding query. In 9.1, queries affected by this bug would fail with "ERROR: Upper-level PlaceHolderVar found where not expected". But in 9.0 and 8.4, you'd silently get possibly-wrong answers, since the value transmitted into the subquery wouldn't go to null when it should.
- Fix planning of UNION ALL subqueries with output columns that are not simple variables Planning of such cases got noticeably worse in 9.1 as a result of a misguided fix for "MergeAppend child's targetlist doesn't match MergeAppend" errors. Revert that fix and do it another way.
- Fix slow session startup when pg_attribute is very large If pg_attribute exceeds one-fourth of shared_buffers, cache rebuilding code that is sometimes needed during session start would trigger the synchronized-scan logic, causing it to take many times longer than normal. The problem was particularly acute if many new sessions were starting at once.
- Ensure sequential scans check for query cancel reasonably often A scan encountering many consecutive pages that contain no live tuples would not respond to interrupts meanwhile.
- Ensure the Windows implementation of PGSemaphoreLock() clears ImmediateInterruptOK before returning This oversight meant that a query-cancel interrupt received later in the same query could be accepted at an unsafe time, with unpredictable but not good consequences.
- Show whole-row variables safely when printing views or rules Corner cases involving ambiguous names (that is, the name could be either a table or column name of the query) were printed in an ambiguous way, risking that the view or rule would be interpreted differently after dump and reload. Avoid the ambiguous case by attaching a no-op cast.
- Fix COPY FROM to properly handle null marker strings that correspond to invalid encoding A null marker string such as E'\0' should work, and did work in the past, but the case got broken in 8.4.
- Fix EXPLAIN VERBOSE for writable CTEs containing RETURNING clauses 
- Fix PREPARE TRANSACTION to work correctly in the presence of advisory locks Historically, PREPARE TRANSACTION has simply ignored any session-level advisory locks the session holds, but this case was accidentally broken in 9.1.
- Fix truncation of unlogged tables 
- Ignore missing schemas during non-interactive assignments of search_path This re-aligns 9.1's behavior with that of older branches. Previously 9.1 would throw an error for nonexistent schemas mentioned in search_path settings obtained from places such as ALTER DATABASE SET.
- Fix bugs with temporary or transient tables used in extension scripts This includes cases such as a rewriting ALTER TABLE within an extension update script, since that uses a transient table behind the scenes.
- Ensure autovacuum worker processes perform stack depth checking properly Previously, infinite recursion in a function invoked by auto-ANALYZE could crash worker processes.
- Fix logging collector to not lose log coherency under high load The collector previously could fail to reassemble large messages if it got too busy.
- Fix logging collector to ensure it will restart file rotation after receiving SIGHUP 
- Fix "too many LWLocks taken" failure in GiST indexes 
- Fix WAL replay logic for GIN indexes to not fail if the index was subsequently dropped 
- Correctly detect SSI conflicts of prepared transactions after a crash 
- Avoid synchronous replication delay when committing a transaction that only modified temporary tables In such a case the transaction's commit record need not be flushed to standby servers, but some of the code didn't know that and waited for it to happen anyway.
- Fix error handling in pg_basebackup 
- Fix walsender to not go into a busy loop if connection is terminated 
- Fix memory leak in PL/pgSQL's RETURN NEXT command 
- Fix PL/pgSQL's GET DIAGNOSTICS command when the target is the function's first variable 
- Ensure that PL/Perl package-qualifies the _TD variable This bug caused trigger invocations to fail when they are nested within a function invocation that changes the current package.
- Fix PL/Python functions returning composite types to accept a string for their result value This case was accidentally broken by the 9.1 additions to allow a composite result value to be supplied in other formats, such as dictionaries.
- Fix potential access off the end of memory in psql's expanded display (x) mode 
- Fix several performance problems in pg_dump when the database contains many objects pg_dump could get very slow if the database contained many schemas, or if many objects are in dependency loops, or if there are many owned sequences.
- Fix memory and file descriptor leaks in pg_restore when reading a directory-format archive 
- Fix pg_upgrade for the case that a database stored in a non-default tablespace contains a table in the cluster's default tablespace 
- In ecpg, fix rare memory leaks and possible overwrite of one byte after the sqlca_t structure 
- Fix contrib/dblink's dblink_exec() to not leak temporary database connections upon error 
- Fix contrib/dblink to report the correct connection name in error messages 
- Fix contrib/vacuumlo to use multiple transactions when dropping many large objects This change avoids exceeding max_locks_per_transaction when many objects need to be dropped. The behavior can be adjusted with the new -l (limit) option.
- Update time zone data files to tzdata release 2012c for DST law changes in Antarctica, Armenia, Chile, Cuba, Falkland Islands, Gaza, Haiti, Hebron, Morocco, Syria, and Tokelau Islands; also historical corrections for Canada.