path: root/src/backend/utils/sort/tuplestore.c

/*-------------------------------------------------------------------------
 *
 * tuplestore.c
 *	  Generalized routines for temporary tuple storage.
 *
 * This module handles temporary storage of tuples for purposes such
 * as Materialize nodes, hashjoin batch files, etc.  It is essentially
 * a dumbed-down version of tuplesort.c; it does no sorting of tuples
 * but can only store and regurgitate a sequence of tuples.  However,
 * because no sort is required, it is allowed to start reading the sequence
 * before it has all been written.	This is particularly useful for cursors,
 * because it allows random access within the already-scanned portion of
 * a query without having to process the underlying scan to completion.
 * A temporary file is used to handle the data if it exceeds the
 * space limit specified by the caller.
 *
 * The (approximate) amount of memory allowed to the tuplestore is specified
 * in kilobytes by the caller.	We absorb tuples and simply store them in an
 * in-memory array as long as we haven't exceeded maxKBytes.  If we do exceed
 * maxKBytes, we dump all the tuples into a temp file and then read from that
 * when needed.
 *
 * When the caller requests random access to the data, we write the temp file
 * in a format that allows either forward or backward scan.  Otherwise, only
 * forward scan is allowed.  But rewind and markpos/restorepos are allowed
 * in any case.
 *
 * Because we allow reading before writing is complete, there are two
 * interesting positions in the temp file: the current read position and
 * the current write position.	At any given instant, the temp file's seek
 * position corresponds to one of these, and the other one is remembered in
 * the Tuplestore's state.
 *
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/utils/sort/tuplestore.c,v 1.18 2004/02/03 17:34:03 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/heapam.h"
#include "storage/buffile.h"
#include "utils/tuplestore.h"

/*
 * Possible states of a Tuplestore object.	These denote the states that
 * persist between calls of Tuplestore routines.
 */
typedef enum
{
	TSS_INMEM,					/* Tuples still fit in memory */
	TSS_WRITEFILE,				/* Writing to temp file */
	TSS_READFILE				/* Reading from temp file */
} TupStoreStatus;

/*
 * Private state of a Tuplestore operation.
 */
struct Tuplestorestate
{
	TupStoreStatus status;		/* enumerated value as shown above */
	bool		randomAccess;	/* did caller request random access? */
	bool		interXact;		/* keep open through transactions? */
	long		availMem;		/* remaining memory available, in bytes */
	BufFile    *myfile;			/* underlying file, or NULL if none */

	/*
	 * These function pointers decouple the routines that must know what
	 * kind of tuple we are handling from the routines that don't need to
	 * know it. They are set up by the tuplestore_begin_xxx routines.
	 *
	 * (Although tuplestore.c currently only supports heap tuples, I've
	 * copied this part of tuplesort.c so that extension to other kinds of
	 * objects will be easy if it's ever needed.)
	 *
	 * Function to copy a supplied input tuple into palloc'd space. (NB: we
	 * assume that a single pfree() is enough to release the tuple later,
	 * so the representation must be "flat" in one palloc chunk.)
	 * state->availMem must be decreased by the amount of space used.
	 */
	void	   *(*copytup) (Tuplestorestate *state, void *tup);

	/*
	 * Function to write a stored tuple onto tape.	The representation of
	 * the tuple on tape need not be the same as it is in memory;
	 * requirements on the tape representation are given below.  After
	 * writing the tuple, pfree() it, and increase state->availMem by the
	 * amount of memory space thereby released.
	 */
	void		(*writetup) (Tuplestorestate *state, void *tup);

	/*
	 * Function to read a stored tuple from tape back into memory. 'len'
	 * is the already-read length of the stored tuple.	Create and return
	 * a palloc'd copy, and decrease state->availMem by the amount of
	 * memory space consumed.
	 */
	void	   *(*readtup) (Tuplestorestate *state, unsigned int len);

	/*
	 * This array holds pointers to tuples in memory if we are in state
	 * INMEM.	In states WRITEFILE and READFILE it's not used.
	 */
	void	  **memtuples;		/* array of pointers to palloc'd tuples */
	int			memtupcount;	/* number of tuples currently present */
	int			memtupsize;		/* allocated length of memtuples array */

	/*
	 * These variables are used to keep track of the current position.
	 *
	 * In state WRITEFILE, the current file seek position is the write point,
	 * and the read position is remembered in readpos_xxx; in state
	 * READFILE, the current file seek position is the read point, and the
	 * write position is remembered in writepos_xxx.  (The write position
	 * is the same as EOF, but since BufFileSeek doesn't currently
	 * implement SEEK_END, we have to remember it explicitly.)
	 *
	 * Special case: if we are in WRITEFILE state and eof_reached is true,
	 * then the read position is implicitly equal to the write position
	 * (and hence to the file seek position); this way we need not update
	 * the readpos_xxx variables on each write.
	 */
	bool		eof_reached;	/* read reached EOF (always valid) */
	int			current;		/* next array index (valid if INMEM) */
	int			readpos_file;	/* file# (valid if WRITEFILE and not eof) */
	long		readpos_offset; /* offset (valid if WRITEFILE and not eof) */
	int			writepos_file;	/* file# (valid if READFILE) */
	long		writepos_offset;	/* offset (valid if READFILE) */

	/* markpos_xxx holds marked position for mark and restore */
	int			markpos_current;	/* saved "current" */
	int			markpos_file;	/* saved "readpos_file" */
	long		markpos_offset; /* saved "readpos_offset" */
};

#define COPYTUP(state,tup)	((*(state)->copytup) (state, tup))
#define WRITETUP(state,tup) ((*(state)->writetup) (state, tup))
#define READTUP(state,len)	((*(state)->readtup) (state, len))
#define LACKMEM(state)		((state)->availMem < 0)
#define USEMEM(state,amt)	((state)->availMem -= (amt))
#define FREEMEM(state,amt)	((state)->availMem += (amt))

/*--------------------
 *
 * NOTES about on-tape representation of tuples:
 *
 * We require the first "unsigned int" of a stored tuple to be the total size
 * on-tape of the tuple, including itself (so it is never zero).
 * The remainder of the stored tuple
 * may or may not match the in-memory representation of the tuple ---
 * any conversion needed is the job of the writetup and readtup routines.
 *
 * If state->randomAccess is true, then the stored representation of the
 * tuple must be followed by another "unsigned int" that is a copy of the
 * length --- so the total tape space used is actually sizeof(unsigned int)
 * more than the stored length value.  This allows read-backwards.	When
 * randomAccess is not true, the write/read routines may omit the extra
 * length word.
 *
 * writetup is expected to write both length words as well as the tuple
 * data.  When readtup is called, the tape is positioned just after the
 * front length word; readtup must read the tuple data and advance past
 * the back length word (if present).
 *
 * The write/read routines can make use of the tuple description data
 * stored in the Tuplestorestate record, if needed. They are also expected
 * to adjust state->availMem by the amount of memory space (not tape space!)
 * released or consumed.  There is no error return from either writetup
 * or readtup; they should ereport() on failure.
 *
 *
 * NOTES about memory consumption calculations:
 *
 * We count space allocated for tuples against the maxKBytes limit,
 * plus the space used by the variable-size array memtuples.
 * Fixed-size space (primarily the BufFile I/O buffer) is not counted.
 *
 * Note that we count actual space used (as shown by GetMemoryChunkSpace)
 * rather than the originally-requested size.  This is important since
 * palloc can add substantial overhead.  It's not a complete answer since
 * we won't count any wasted space in palloc allocation blocks, but it's
 * a lot better than what we were doing before 7.3.
 *
 *--------------------
 */


static Tuplestorestate *tuplestore_begin_common(bool randomAccess,
						bool interXact,
						int maxKBytes);
static void dumptuples(Tuplestorestate *state);
static unsigned int getlen(Tuplestorestate *state, bool eofOK);
static void *copytup_heap(Tuplestorestate *state, void *tup);
static void writetup_heap(Tuplestorestate *state, void *tup);
static void *readtup_heap(Tuplestorestate *state, unsigned int len);


/*
 *		tuplestore_begin_xxx
 *
 * Initialize for a tuple store operation.
 */
static Tuplestorestate *
tuplestore_begin_common(bool randomAccess, bool interXact, int maxKBytes)
{
	Tuplestorestate *state;

	state = (Tuplestorestate *) palloc0(sizeof(Tuplestorestate));

	state->status = TSS_INMEM;
	state->randomAccess = randomAccess;
	state->interXact = interXact;
	state->availMem = maxKBytes * 1024L;
	state->myfile = NULL;

	state->memtupcount = 0;
	state->memtupsize = 1024;					/* initial guess */
	state->memtuples = (void **) palloc(state->memtupsize * sizeof(void *));

	USEMEM(state, GetMemoryChunkSpace(state->memtuples));

	state->eof_reached = false;
	state->current = 0;

	return state;
}

/*
 * tuplestore_begin_heap
 *
 * Create a new tuplestore; other types of tuple stores (other than
 * "heap" tuple stores, for heap tuples) are possible, but not presently
 * implemented.
 *
 * randomAccess: if true, both forward and backward accesses to the
 * tuple store are allowed.
 *
 * interXact: if true, the files used for on-disk storage persist beyond the
 * end of the current transaction.	NOTE: It's the caller's responsibility to
 * create such a tuplestore in a memory context that will also survive
 * transaction boundaries, and to ensure the tuplestore is closed when it's
 * no longer wanted.
 *
 * maxKBytes: how much data to store in memory (any data beyond this
 * amount is paged to disk).  When in doubt, use work_mem.
 */
Tuplestorestate *
tuplestore_begin_heap(bool randomAccess, bool interXact, int maxKBytes)
{
	Tuplestorestate *state = tuplestore_begin_common(randomAccess,
													 interXact,
													 maxKBytes);

	state->copytup = copytup_heap;
	state->writetup = writetup_heap;
	state->readtup = readtup_heap;

	return state;
}

/*
 * tuplestore_end
 *
 *	Release resources and clean up.
 */
void
tuplestore_end(Tuplestorestate *state)
{
	int			i;

	if (state->myfile)
		BufFileClose(state->myfile);
	if (state->memtuples)
	{
		for (i = 0; i < state->memtupcount; i++)
			pfree(state->memtuples[i]);
		pfree(state->memtuples);
	}
}

/*
 * tuplestore_ateof
 *
 * Returns the current eof_reached state.
 */
bool
tuplestore_ateof(Tuplestorestate *state)
{
	return state->eof_reached;
}

/*
 * Accept one tuple and append it to the tuplestore.
 *
 * Note that the input tuple is always copied; the caller need not save it.
 *
 * If the read status is currently "AT EOF" then it remains so (the read
 * pointer advances along with the write pointer); otherwise the read
 * pointer is unchanged.  This is for the convenience of nodeMaterial.c.
 */
void
tuplestore_puttuple(Tuplestorestate *state, void *tuple)
{
	/*
	 * Copy the tuple.	(Must do this even in WRITEFILE case.)
	 */
	tuple = COPYTUP(state, tuple);

	switch (state->status)
	{
		case TSS_INMEM:
			/* Grow the array as needed */
			if (state->memtupcount >= state->memtupsize)
			{
				FREEMEM(state, GetMemoryChunkSpace(state->memtuples));
				state->memtupsize *= 2;
				state->memtuples = (void **)
					repalloc(state->memtuples,
							 state->memtupsize * sizeof(void *));
				USEMEM(state, GetMemoryChunkSpace(state->memtuples));
			}

			/* Stash the tuple in the in-memory array */
			state->memtuples[state->memtupcount++] = tuple;

			/* If eof_reached, keep read position in sync */
			if (state->eof_reached)
				state->current = state->memtupcount;

			/*
			 * Done if we still fit in available memory.
			 */
			if (!LACKMEM(state))
				return;

			/*
			 * Nope; time to switch to tape-based operation.
			 */
			state->myfile = BufFileCreateTemp(state->interXact);
			state->status = TSS_WRITEFILE;
			dumptuples(state);
			break;
		case TSS_WRITEFILE:
			WRITETUP(state, tuple);
			break;
		case TSS_READFILE:

			/*
			 * Switch from reading to writing.
			 */
			if (!state->eof_reached)
				BufFileTell(state->myfile,
							&state->readpos_file, &state->readpos_offset);
			if (BufFileSeek(state->myfile,
							state->writepos_file, state->writepos_offset,
							SEEK_SET) != 0)
				elog(ERROR, "seek to EOF failed");
			state->status = TSS_WRITEFILE;
			WRITETUP(state, tuple);
			break;
		default:
			elog(ERROR, "invalid tuplestore state");
			break;
	}
}

/*
 * Fetch the next tuple in either forward or back direction.
 * Returns NULL if no more tuples.	If should_free is set, the
 * caller must pfree the returned tuple when done with it.
 */
void *
tuplestore_gettuple(Tuplestorestate *state, bool forward,
					bool *should_free)
{
	unsigned int tuplen;
	void	   *tup;

	Assert(forward || state->randomAccess);

	switch (state->status)
	{
		case TSS_INMEM:
			*should_free = false;
			if (forward)
			{
				if (state->current < state->memtupcount)
					return state->memtuples[state->current++];
				state->eof_reached = true;
				return NULL;
			}
			else
			{
				if (state->current <= 0)
					return NULL;

				/*
				 * if all tuples are fetched already then we return last
				 * tuple, else - tuple before last returned.
				 */
				if (state->eof_reached)
					state->eof_reached = false;
				else
				{
					state->current--;	/* last returned tuple */
					if (state->current <= 0)
						return NULL;
				}
				return state->memtuples[state->current - 1];
			}
			break;

		case TSS_WRITEFILE:
			/* Skip state change if we'll just return NULL */
			if (state->eof_reached && forward)
				return NULL;

			/*
			 * Switch from writing to reading.
			 */
			BufFileTell(state->myfile,
						&state->writepos_file, &state->writepos_offset);
			if (!state->eof_reached)
				if (BufFileSeek(state->myfile,
							  state->readpos_file, state->readpos_offset,
								SEEK_SET) != 0)
					elog(ERROR, "seek failed");
			state->status = TSS_READFILE;
			/* FALL THRU into READFILE case */

		case TSS_READFILE:
			*should_free = true;
			if (forward)
			{
				if ((tuplen = getlen(state, true)) != 0)
				{
					tup = READTUP(state, tuplen);
					return tup;
				}
				else
				{
					state->eof_reached = true;
					return NULL;
				}
			}

			/*
			 * Backward.
			 *
			 * if all tuples are fetched already then we return last tuple,
			 * else - tuple before last returned.
			 *
			 * Back up to fetch previously-returned tuple's ending length
			 * word.  If seek fails, assume we are at start of file.
			 */
			if (BufFileSeek(state->myfile, 0, -(long) sizeof(unsigned int),
							SEEK_CUR) != 0)
				return NULL;
			tuplen = getlen(state, false);

			if (state->eof_reached)
			{
				state->eof_reached = false;
				/* We will return the tuple returned before returning NULL */
			}
			else
			{
				/*
				 * Back up to get ending length word of tuple before it.
				 */
				if (BufFileSeek(state->myfile, 0,
							 -(long) (tuplen + 2 * sizeof(unsigned int)),
								SEEK_CUR) != 0)
				{
					/*
					 * If that fails, presumably the prev tuple is the
					 * first in the file.  Back up so that it becomes next
					 * to read in forward direction (not obviously right,
					 * but that is what in-memory case does).
					 */
					if (BufFileSeek(state->myfile, 0,
								 -(long) (tuplen + sizeof(unsigned int)),
									SEEK_CUR) != 0)
						elog(ERROR, "bogus tuple length in backward scan");
					return NULL;
				}
				tuplen = getlen(state, false);
			}

			/*
			 * Now we have the length of the prior tuple, back up and read
			 * it. Note: READTUP expects we are positioned after the
			 * initial length word of the tuple, so back up to that point.
			 */
			if (BufFileSeek(state->myfile, 0,
							-(long) tuplen,
							SEEK_CUR) != 0)
				elog(ERROR, "bogus tuple length in backward scan");
			tup = READTUP(state, tuplen);
			return tup;

		default:
			elog(ERROR, "invalid tuplestore state");
			return NULL;		/* keep compiler quiet */
	}
}

/*
 * dumptuples - remove tuples from memory and write to tape
 *
 * As a side effect, we must set readpos and markpos to the value
 * corresponding to "current"; otherwise, a dump would lose the current read
 * position.
 */
static void
dumptuples(Tuplestorestate *state)
{
	int			i;

	for (i = 0;; i++)
	{
		if (i == state->current)
			BufFileTell(state->myfile,
						&state->readpos_file, &state->readpos_offset);
		if (i == state->markpos_current)
			BufFileTell(state->myfile,
						&state->markpos_file, &state->markpos_offset);
		if (i >= state->memtupcount)
			break;
		WRITETUP(state, state->memtuples[i]);
	}
	state->memtupcount = 0;
}

/*
 * tuplestore_rescan		- rewind and replay the scan
 */
void
tuplestore_rescan(Tuplestorestate *state)
{
	switch (state->status)
	{
		case TSS_INMEM:
			state->eof_reached = false;
			state->current = 0;
			break;
		case TSS_WRITEFILE:
			state->eof_reached = false;
			state->readpos_file = 0;
			state->readpos_offset = 0L;
			break;
		case TSS_READFILE:
			state->eof_reached = false;
			if (BufFileSeek(state->myfile, 0, 0L, SEEK_SET) != 0)
				elog(ERROR, "seek to start failed");
			break;
		default:
			elog(ERROR, "invalid tuplestore state");
			break;
	}
}

/*
 * tuplestore_markpos	- saves current position in the tuple sequence
 */
void
tuplestore_markpos(Tuplestorestate *state)
{
	switch (state->status)
	{
		case TSS_INMEM:
			state->markpos_current = state->current;
			break;
		case TSS_WRITEFILE:
			if (state->eof_reached)
			{
				/* Need to record the implicit read position */
				BufFileTell(state->myfile,
							&state->markpos_file,
							&state->markpos_offset);
			}
			else
			{
				state->markpos_file = state->readpos_file;
				state->markpos_offset = state->readpos_offset;
			}
			break;
		case TSS_READFILE:
			BufFileTell(state->myfile,
						&state->markpos_file,
						&state->markpos_offset);
			break;
		default:
			elog(ERROR, "invalid tuplestore state");
			break;
	}
}

/*
 * tuplestore_restorepos - restores current position in tuple sequence to
 *						  last saved position
 */
void
tuplestore_restorepos(Tuplestorestate *state)
{
	switch (state->status)
	{
		case TSS_INMEM:
			state->eof_reached = false;
			state->current = state->markpos_current;
			break;
		case TSS_WRITEFILE:
			state->eof_reached = false;
			state->readpos_file = state->markpos_file;
			state->readpos_offset = state->markpos_offset;
			break;
		case TSS_READFILE:
			state->eof_reached = false;
			if (BufFileSeek(state->myfile,
							state->markpos_file,
							state->markpos_offset,
							SEEK_SET) != 0)
				elog(ERROR, "tuplestore_restorepos failed");
			break;
		default:
			elog(ERROR, "invalid tuplestore state");
			break;
	}
}


/*
 * Tape interface routines
 */

static unsigned int
getlen(Tuplestorestate *state, bool eofOK)
{
	unsigned int len;
	size_t		nbytes;

	nbytes = BufFileRead(state->myfile, (void *) &len, sizeof(len));
	if (nbytes == sizeof(len))
		return len;
	if (nbytes != 0)
		elog(ERROR, "unexpected end of tape");
	if (!eofOK)
		elog(ERROR, "unexpected end of data");
	return 0;
}


/*
 * Routines specialized for HeapTuple case
 */

static void *
copytup_heap(Tuplestorestate *state, void *tup)
{
	HeapTuple	tuple = (HeapTuple) tup;

	tuple = heap_copytuple(tuple);
	USEMEM(state, GetMemoryChunkSpace(tuple));
	return (void *) tuple;
}

/*
 * We don't bother to write the HeapTupleData part of the tuple.
 */

static void
writetup_heap(Tuplestorestate *state, void *tup)
{
	HeapTuple	tuple = (HeapTuple) tup;
	unsigned int tuplen;

	tuplen = tuple->t_len + sizeof(tuplen);
	if (BufFileWrite(state->myfile, (void *) &tuplen,
					 sizeof(tuplen)) != sizeof(tuplen))
		elog(ERROR, "write failed");
	if (BufFileWrite(state->myfile, (void *) tuple->t_data,
					 tuple->t_len) != (size_t) tuple->t_len)
		elog(ERROR, "write failed");
	if (state->randomAccess)	/* need trailing length word? */
		if (BufFileWrite(state->myfile, (void *) &tuplen,
						 sizeof(tuplen)) != sizeof(tuplen))
			elog(ERROR, "write failed");

	FREEMEM(state, GetMemoryChunkSpace(tuple));
	heap_freetuple(tuple);
}

static void *
readtup_heap(Tuplestorestate *state, unsigned int len)
{
	unsigned int tuplen = len - sizeof(unsigned int) + HEAPTUPLESIZE;
	HeapTuple	tuple = (HeapTuple) palloc(tuplen);

	USEMEM(state, GetMemoryChunkSpace(tuple));
	/* reconstruct the HeapTupleData portion */
	tuple->t_len = len - sizeof(unsigned int);
	ItemPointerSetInvalid(&(tuple->t_self));
	tuple->t_datamcxt = CurrentMemoryContext;
	tuple->t_data = (HeapTupleHeader) (((char *) tuple) + HEAPTUPLESIZE);
	/* read in the tuple proper */
	if (BufFileRead(state->myfile, (void *) tuple->t_data,
					tuple->t_len) != (size_t) tuple->t_len)
		elog(ERROR, "unexpected end of data");
	if (state->randomAccess)	/* need trailing length word? */
		if (BufFileRead(state->myfile, (void *) &tuplen,
						sizeof(tuplen)) != sizeof(tuplen))
			elog(ERROR, "unexpected end of data");
	return (void *) tuple;
}