This addresses only those cases that are easy to fix by adding or
moving a const qualifier or removing an unnecessary cast.  There are
many more complicated cases remaining.

As pointed out by Sergey Koposov, repeated invocations of tbm_lossify can
make building a large tidbitmap into an O(N^2) operation.  To fix, make
sure we remove more than the minimum amount of information per call, and
add a fallback path to behave sanely if we're unable to fit the bitmap
within the requested amount of memory.

This has been wrong since the tidbitmap code was written, so back-patch
to all supported branches.

provided by Andrew.

multiple index entries in a holding area before adding them to the main index
structure.  This helps because bulk insert is (usually) significantly faster
than retail insert for GIN.

This patch also removes GIN support for amgettuple-style index scans.  The
API defined for amgettuple is difficult to support with fastupdate, and
the previously committed partial-match feature didn't really work with
it either.  We might eventually figure a way to put back amgettuple
support, but it won't happen for 8.4.

catversion bumped because of change in GIN's pg_am entry, and because
the format of GIN indexes changed on-disk (there's a metapage now,
and possibly a pending list).

Teodor Sigaev

bitmap.  This is extracted from Greg Stark's posix_fadvise patch; it seems
worth committing separately, since it's potentially useful independently of
posix_fadvise.

indexscan always occurs in one call, and the results are returned in a
TIDBitmap instead of a limited-size array of TIDs.  This should improve
speed a little by reducing AM entry/exit overhead, and it is necessary
infrastructure if we are ever to support bitmap indexes.

In an only slightly related change, add support for TIDBitmaps to preserve
(somewhat lossily) the knowledge that particular TIDs reported by an index
need to have their quals rechecked when the heap is visited.  This facility
is not really used yet; we'll need to extend the forced-recheck feature to
plain indexscans before it's useful, and that hasn't been coded yet.
The intent is to use it to clean up 8.3's horrid @@@ kluge for text search
with weighted queries.  There might be other uses in future, but that one
alone is sufficient reason.

Heikki Linnakangas, with some adjustments by me.

columns, and the new version can be stored on the same heap page, we no longer
generate extra index entries for the new version.  Instead, index searches
follow the HOT-chain links to ensure they find the correct tuple version.

In addition, this patch introduces the ability to "prune" dead tuples on a
per-page basis, without having to do a complete VACUUM pass to recover space.
VACUUM is still needed to clean up dead index entries, however.

Pavan Deolasee, with help from a bunch of other people.

is in progress on the same hashtable.  This seems the least invasive way to
fix the recently-recognized problem that a split could cause the scan to
visit entries twice or (with much lower probability) miss them entirely.
The only field-reported problem caused by this is the "failed to re-find
shared lock object" PANIC in COMMIT PREPARED reported by Michel Dorochevsky,
which was caused by multiply visited entries.  However, it seems certain
that mdsync() is vulnerable to missing required fsync's due to missed
entries, and I am fearful that RelationCacheInitializePhase2() might be at
risk as well.  Because of that and the generalized hazard presented by this
bug, back-patch all the supported branches.

Along the way, fix pg_prepared_statement() and pg_cursor() to not assume
that the hashtables they are examining will stay static between calls.
This is risky regardless of the newly noted dynahash problem, because
hash_seq_search() has never promised to cope with deletion of table entries
other than the just-returned one.  There may be no bug here because the only
supported way to call these functions is via ExecMakeTableFunctionResult()
which will cycle them to completion before doing anything very interesting,
but it seems best to get rid of the assumption.  This affects 8.2 and HEAD
only, since those functions weren't there earlier.

back-stamped for this.

on a page, as suggested by ITAGAKI Takahiro.  Also, change a few places
that were using some other estimates of max-items-per-page to consistently
use MaxOffsetNumber.  This is conservatively large --- we could have used
the new MaxHeapTuplesPerPage macro, or a similar one for index tuples ---
but those places are simply declaring a fixed-size buffer and assuming it
will work, rather than actively testing for overrun.  It seems safer to
size these buffers in a way that can't overflow even if the page is
corrupt.

got an empty bitmap after any step; the remaining subplans can no longer
affect the result.  Per a suggestion from Ilia Kantor.

a lossy page has to be lossy, because we don't know exactly which tuples
on the page should remain part of the bitmap.  Per Jie Zhang.

spotted by Qingqing Zhou.  The HASH_ENTER action now automatically
fails with elog(ERROR) on out-of-memory --- which incidentally lets
us eliminate duplicate error checks in quite a bunch of places.  If
you really need the old return-NULL-on-out-of-memory behavior, you
can ask for HASH_ENTER_NULL.  But there is now an Assert in that path
checking that you aren't hoping to get that behavior in a palloc-based
hash table.
Along the way, remove the old HASH_FIND_SAVE/HASH_REMOVE_SAVED actions,
which were not being used anywhere anymore, and were surely too ugly
and unsafe to want to see revived again.

than one heap page represented in the bitmap.  This is a bit ugly but
it cuts overhead fairly effectively in simple join cases.  Per example
from Sergey Koposov.

scans, using in-memory tuple ID bitmaps as the intermediary.  The planner
frontend (path creation and cost estimation) is not there yet, so none
of this code can be executed.  I have tested it using some hacked planner
code that is far too ugly to see the light of day, however.  Committing
now so that the bulk of the infrastructure changes go in before the tree
drifts under me.

Not connected to anything useful yet ...