path: root/src/backend/access/heap/tuptoaster.c

/*-------------------------------------------------------------------------
 *
 * tuptoaster.c
 *	  Support routines for external and compressed storage of
 *	  variable size attributes.
 *
 * Copyright (c) 2000, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/access/heap/tuptoaster.c,v 1.25 2001/10/25 05:49:21 momjian Exp $
 *
 *
 * INTERFACE ROUTINES
 *		heap_tuple_toast_attrs -
 *			Try to make a given tuple fit into one page by compressing
 *			or moving off attributes
 *
 *		heap_tuple_untoast_attr -
 *			Fetch back a given value from the "secondary" relation
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>

#include "access/heapam.h"
#include "access/genam.h"
#include "access/tuptoaster.h"
#include "catalog/catalog.h"
#include "utils/rel.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/pg_lzcompress.h"


#ifdef TUPLE_TOASTER_ACTIVE

#undef TOAST_DEBUG

static void toast_delete(Relation rel, HeapTuple oldtup);
static void toast_delete_datum(Relation rel, Datum value);
static void toast_insert_or_update(Relation rel, HeapTuple newtup,
					   HeapTuple oldtup);
static Datum toast_save_datum(Relation rel, Datum value);
static varattrib *toast_fetch_datum(varattrib *attr);


/* ----------
 * heap_tuple_toast_attrs -
 *
 *	This is the central public entry point for toasting from heapam.
 *
 *	Calls the appropriate event specific action.
 * ----------
 */
void
heap_tuple_toast_attrs(Relation rel, HeapTuple newtup, HeapTuple oldtup)
{
	if (newtup == NULL)
		toast_delete(rel, oldtup);
	else
		toast_insert_or_update(rel, newtup, oldtup);
}


/* ----------
 * heap_tuple_fetch_attr -
 *
 *	Public entry point to get back a toasted value
 *	external storage (possibly still in compressed format).
 * ----------
 */
varattrib *
heap_tuple_fetch_attr(varattrib *attr)
{
	varattrib  *result;

	if (VARATT_IS_EXTERNAL(attr))
	{
		/*
		 * This is an external stored plain value
		 */
		result = toast_fetch_datum(attr);
	}
	else
	{
		/*
		 * This is a plain value inside of the main tuple - why am I
		 * called?
		 */
		result = attr;
	}

	return result;
}


/* ----------
 * heap_tuple_untoast_attr -
 *
 *	Public entry point to get back a toasted value from compression
 *	or external storage.
 * ----------
 */
varattrib *
heap_tuple_untoast_attr(varattrib *attr)
{
	varattrib  *result;

	if (VARATT_IS_EXTERNAL(attr))
	{
		if (VARATT_IS_COMPRESSED(attr))
		{
			/* ----------
			 * This is an external stored compressed value
			 * Fetch it from the toast heap and decompress.
			 * ----------
			 */
			varattrib  *tmp;

			tmp = toast_fetch_datum(attr);
			result = (varattrib *) palloc(attr->va_content.va_external.va_rawsize
										  + VARHDRSZ);
			VARATT_SIZEP(result) = attr->va_content.va_external.va_rawsize
				+ VARHDRSZ;
			pglz_decompress((PGLZ_Header *) tmp, VARATT_DATA(result));

			pfree(tmp);
		}
		else
		{
			/*
			 * This is an external stored plain value
			 */
			result = toast_fetch_datum(attr);
		}
	}
	else if (VARATT_IS_COMPRESSED(attr))
	{
		/*
		 * This is a compressed value inside of the main tuple
		 */
		result = (varattrib *) palloc(attr->va_content.va_compressed.va_rawsize
									  + VARHDRSZ);
		VARATT_SIZEP(result) = attr->va_content.va_compressed.va_rawsize
			+ VARHDRSZ;
		pglz_decompress((PGLZ_Header *) attr, VARATT_DATA(result));
	}
	else

		/*
		 * This is a plain value inside of the main tuple - why am I
		 * called?
		 */
		return attr;

	return result;
}


/* ----------
 * toast_raw_datum_size -
 *
 *	Return the raw (detoasted) size of a varlena datum
 * ----------
 */
Size
toast_raw_datum_size(Datum value)
{
	varattrib  *attr = (varattrib *) DatumGetPointer(value);
	Size		result;

	if (VARATT_IS_COMPRESSED(attr))
	{
		/*
		 * va_rawsize shows the original data size, whether the datum is
		 * external or not.
		 */
		result = attr->va_content.va_compressed.va_rawsize + VARHDRSZ;
	}
	else if (VARATT_IS_EXTERNAL(attr))
	{
		/*
		 * an uncompressed external attribute has rawsize including the
		 * header (not too consistent!)
		 */
		result = attr->va_content.va_external.va_rawsize;
	}
	else
	{
		/* plain untoasted datum */
		result = VARSIZE(attr);
	}
	return result;
}


/* ----------
 * toast_delete -
 *
 *	Cascaded delete toast-entries on DELETE
 * ----------
 */
static void
toast_delete(Relation rel, HeapTuple oldtup)
{
	TupleDesc	tupleDesc;
	Form_pg_attribute *att;
	int			numAttrs;
	int			i;
	Datum		value;
	bool		isnull;

	/*
	 * Get the tuple descriptor, the number of and attribute descriptors.
	 */
	tupleDesc = rel->rd_att;
	numAttrs = tupleDesc->natts;
	att = tupleDesc->attrs;

	/*
	 * Check for external stored attributes and delete them from the
	 * secondary relation.
	 */
	for (i = 0; i < numAttrs; i++)
	{
		if (att[i]->attlen == -1)
		{
			value = heap_getattr(oldtup, i + 1, tupleDesc, &isnull);
			if (!isnull && VARATT_IS_EXTERNAL(value))
				toast_delete_datum(rel, value);
		}
	}
}


/* ----------
 * toast_insert_or_update -
 *
 *	Delete no-longer-used toast-entries and create new ones to
 *	make the new tuple fit on INSERT or UPDATE
 * ----------
 */
static void
toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
{
	TupleDesc	tupleDesc;
	Form_pg_attribute *att;
	int			numAttrs;
	int			i;
	bool		old_isnull;
	bool		new_isnull;

	bool		need_change = false;
	bool		need_free = false;
	bool		need_delold = false;
	bool		has_nulls = false;

	Size		maxDataLen;

	char		toast_action[MaxHeapAttributeNumber];
	char		toast_nulls[MaxHeapAttributeNumber];
	Datum		toast_values[MaxHeapAttributeNumber];
	int32		toast_sizes[MaxHeapAttributeNumber];
	bool		toast_free[MaxHeapAttributeNumber];
	bool		toast_delold[MaxHeapAttributeNumber];

	/*
	 * Get the tuple descriptor, the number of and attribute descriptors
	 * and the location of the tuple values.
	 */
	tupleDesc = rel->rd_att;
	numAttrs = tupleDesc->natts;
	att = tupleDesc->attrs;

	/* ----------
	 * Then collect information about the values given
	 *
	 * NOTE: toast_action[i] can have these values:
	 *		' '		default handling
	 *		'p'		already processed --- don't touch it
	 *		'x'		incompressible, but OK to move off
	 * ----------
	 */
	memset(toast_action, ' ', numAttrs * sizeof(char));
	memset(toast_nulls, ' ', numAttrs * sizeof(char));
	memset(toast_free, 0, numAttrs * sizeof(bool));
	memset(toast_delold, 0, numAttrs * sizeof(bool));
	for (i = 0; i < numAttrs; i++)
	{
		varattrib  *old_value;
		varattrib  *new_value;

		if (oldtup != NULL)
		{
			/*
			 * For UPDATE get the old and new values of this attribute
			 */
			old_value = (varattrib *) DatumGetPointer(
					heap_getattr(oldtup, i + 1, tupleDesc, &old_isnull));
			toast_values[i] =
				heap_getattr(newtup, i + 1, tupleDesc, &new_isnull);
			new_value = (varattrib *) DatumGetPointer(toast_values[i]);

			/*
			 * If the old value is an external stored one, check if it has
			 * changed so we have to delete it later.
			 */
			if (!old_isnull && att[i]->attlen == -1 &&
				VARATT_IS_EXTERNAL(old_value))
			{
				if (new_isnull || !VARATT_IS_EXTERNAL(new_value) ||
					old_value->va_content.va_external.va_valueid !=
					new_value->va_content.va_external.va_valueid ||
					old_value->va_content.va_external.va_toastrelid !=
					new_value->va_content.va_external.va_toastrelid)
				{
					/*
					 * The old external store value isn't needed any more
					 * after the update
					 */
					toast_delold[i] = true;
					need_delold = true;
				}
				else
				{
					/*
					 * This attribute isn't changed by this update so we
					 * reuse the original reference to the old value in
					 * the new tuple.
					 */
					toast_action[i] = 'p';
					toast_sizes[i] = VARATT_SIZE(toast_values[i]);
					continue;
				}
			}
		}
		else
		{
			/*
			 * For INSERT simply get the new value
			 */
			toast_values[i] =
				heap_getattr(newtup, i + 1, tupleDesc, &new_isnull);
		}

		/*
		 * Handle NULL attributes
		 */
		if (new_isnull)
		{
			toast_action[i] = 'p';
			toast_nulls[i] = 'n';
			has_nulls = true;
			continue;
		}

		/*
		 * Now look at varsize attributes
		 */
		if (att[i]->attlen == -1)
		{
			/*
			 * If the table's attribute says PLAIN always, force it so.
			 */
			if (att[i]->attstorage == 'p')
				toast_action[i] = 'p';

			/*
			 * We took care of UPDATE above, so any external value we find
			 * still in the tuple must be someone else's we cannot reuse.
			 * Expand it to plain (and, probably, toast it again below).
			 */
			if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
			{
				toast_values[i] = PointerGetDatum(heap_tuple_untoast_attr(
						(varattrib *) DatumGetPointer(toast_values[i])));
				toast_free[i] = true;
				need_change = true;
				need_free = true;
			}

			/*
			 * Remember the size of this attribute
			 */
			toast_sizes[i] = VARATT_SIZE(DatumGetPointer(toast_values[i]));
		}
		else
		{
			/*
			 * Not a variable size attribute, plain storage always
			 */
			toast_action[i] = 'p';
			toast_sizes[i] = att[i]->attlen;
		}
	}

	/* ----------
	 * Compress and/or save external until data fits into target length
	 *
	 *	1: Inline compress attributes with attstorage 'x'
	 *	2: Store attributes with attstorage 'x' or 'e' external
	 *	3: Inline compress attributes with attstorage 'm'
	 *	4: Store attributes with attstorage 'm' external
	 * ----------
	 */
	maxDataLen = offsetof(HeapTupleHeaderData, t_bits);
	if (has_nulls)
		maxDataLen += BITMAPLEN(numAttrs);
	maxDataLen = TOAST_TUPLE_TARGET - MAXALIGN(maxDataLen);

	/*
	 * Look for attributes with attstorage 'x' to compress
	 */
	while (MAXALIGN(ComputeDataSize(tupleDesc, toast_values, toast_nulls)) >
		   maxDataLen)
	{
		int			biggest_attno = -1;
		int32		biggest_size = MAXALIGN(sizeof(varattrib));
		Datum		old_value;
		Datum		new_value;

		/*
		 * Search for the biggest yet uncompressed internal attribute
		 */
		for (i = 0; i < numAttrs; i++)
		{
			if (toast_action[i] != ' ')
				continue;
			if (VARATT_IS_EXTENDED(toast_values[i]))
				continue;
			if (att[i]->attstorage != 'x')
				continue;
			if (toast_sizes[i] > biggest_size)
			{
				biggest_attno = i;
				biggest_size = toast_sizes[i];
			}
		}

		if (biggest_attno < 0)
			break;

		/*
		 * Attempt to compress it inline
		 */
		i = biggest_attno;
		old_value = toast_values[i];
		new_value = toast_compress_datum(old_value);

		if (DatumGetPointer(new_value) != NULL)
		{
			/* successful compression */
			if (toast_free[i])
				pfree(DatumGetPointer(old_value));
			toast_values[i] = new_value;
			toast_free[i] = true;
			toast_sizes[i] = VARATT_SIZE(toast_values[i]);
			need_change = true;
			need_free = true;
		}
		else
		{
			/*
			 * incompressible data, ignore on subsequent compression
			 * passes
			 */
			toast_action[i] = 'x';
		}
	}

	/*
	 * Second we look for attributes of attstorage 'x' or 'e' that are
	 * still inline.
	 */
	while (MAXALIGN(ComputeDataSize(tupleDesc, toast_values, toast_nulls)) >
		   maxDataLen && rel->rd_rel->reltoastrelid != InvalidOid)
	{
		int			biggest_attno = -1;
		int32		biggest_size = MAXALIGN(sizeof(varattrib));
		Datum		old_value;

		/*------
		 * Search for the biggest yet inlined attribute with
		 * attstorage equals 'x' or 'e'
		 *------
		 */
		for (i = 0; i < numAttrs; i++)
		{
			if (toast_action[i] == 'p')
				continue;
			if (VARATT_IS_EXTERNAL(toast_values[i]))
				continue;
			if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
				continue;
			if (toast_sizes[i] > biggest_size)
			{
				biggest_attno = i;
				biggest_size = toast_sizes[i];
			}
		}

		if (biggest_attno < 0)
			break;

		/*
		 * Store this external
		 */
		i = biggest_attno;
		old_value = toast_values[i];
		toast_action[i] = 'p';
		toast_values[i] = toast_save_datum(rel, toast_values[i]);
		if (toast_free[i])
			pfree(DatumGetPointer(old_value));

		toast_free[i] = true;
		toast_sizes[i] = VARATT_SIZE(toast_values[i]);

		need_change = true;
		need_free = true;
	}

	/*
	 * Round 3 - this time we take attributes with storage 'm' into
	 * compression
	 */
	while (MAXALIGN(ComputeDataSize(tupleDesc, toast_values, toast_nulls)) >
		   maxDataLen)
	{
		int			biggest_attno = -1;
		int32		biggest_size = MAXALIGN(sizeof(varattrib));
		Datum		old_value;
		Datum		new_value;

		/*
		 * Search for the biggest yet uncompressed internal attribute
		 */
		for (i = 0; i < numAttrs; i++)
		{
			if (toast_action[i] != ' ')
				continue;
			if (VARATT_IS_EXTENDED(toast_values[i]))
				continue;
			if (att[i]->attstorage != 'm')
				continue;
			if (toast_sizes[i] > biggest_size)
			{
				biggest_attno = i;
				biggest_size = toast_sizes[i];
			}
		}

		if (biggest_attno < 0)
			break;

		/*
		 * Attempt to compress it inline
		 */
		i = biggest_attno;
		old_value = toast_values[i];
		new_value = toast_compress_datum(old_value);

		if (DatumGetPointer(new_value) != NULL)
		{
			/* successful compression */
			if (toast_free[i])
				pfree(DatumGetPointer(old_value));
			toast_values[i] = new_value;
			toast_free[i] = true;
			toast_sizes[i] = VARATT_SIZE(toast_values[i]);
			need_change = true;
			need_free = true;
		}
		else
		{
			/*
			 * incompressible data, ignore on subsequent compression
			 * passes
			 */
			toast_action[i] = 'x';
		}
	}

	/*
	 * Finally we store attributes of type 'm' external
	 */
	while (MAXALIGN(ComputeDataSize(tupleDesc, toast_values, toast_nulls)) >
		   maxDataLen && rel->rd_rel->reltoastrelid != InvalidOid)
	{
		int			biggest_attno = -1;
		int32		biggest_size = MAXALIGN(sizeof(varattrib));
		Datum		old_value;

		/*--------
		 * Search for the biggest yet inlined attribute with
		 * attstorage = 'm'
		 *--------
		 */
		for (i = 0; i < numAttrs; i++)
		{
			if (toast_action[i] == 'p')
				continue;
			if (VARATT_IS_EXTERNAL(toast_values[i]))
				continue;
			if (att[i]->attstorage != 'm')
				continue;
			if (toast_sizes[i] > biggest_size)
			{
				biggest_attno = i;
				biggest_size = toast_sizes[i];
			}
		}

		if (biggest_attno < 0)
			break;

		/*
		 * Store this external
		 */
		i = biggest_attno;
		old_value = toast_values[i];
		toast_action[i] = 'p';
		toast_values[i] = toast_save_datum(rel, toast_values[i]);
		if (toast_free[i])
			pfree(DatumGetPointer(old_value));

		toast_free[i] = true;
		toast_sizes[i] = VARATT_SIZE(toast_values[i]);

		need_change = true;
		need_free = true;
	}

	/*
	 * In the case we toasted any values, we need to build a new heap
	 * tuple with the changed values.
	 */
	if (need_change)
	{
		char	   *new_data;
		int32		new_len;
		MemoryContext oldcxt;
		HeapTupleHeader olddata;

		/*
		 * Calculate the new size of the tuple
		 */
		new_len = offsetof(HeapTupleHeaderData, t_bits);
		if (has_nulls)
			new_len += BITMAPLEN(numAttrs);
		new_len = MAXALIGN(new_len);
		new_len += ComputeDataSize(tupleDesc, toast_values, toast_nulls);

		/*
		 * Remember the old memory location of the tuple (for below),
		 * switch to the memory context of the HeapTuple structure and
		 * allocate the new tuple.
		 */
		olddata = newtup->t_data;
		oldcxt = MemoryContextSwitchTo(newtup->t_datamcxt);
		new_data = palloc(new_len);

		/*
		 * Put the tuple header and the changed values into place
		 */
		memcpy(new_data, newtup->t_data, newtup->t_data->t_hoff);
		newtup->t_data = (HeapTupleHeader) new_data;
		newtup->t_len = new_len;

		DataFill((char *) (MAXALIGN((long) new_data +
								  offsetof(HeapTupleHeaderData, t_bits) +
							   ((has_nulls) ? BITMAPLEN(numAttrs) : 0))),
				 tupleDesc,
				 toast_values,
				 toast_nulls,
				 &(newtup->t_data->t_infomask),
				 has_nulls ? newtup->t_data->t_bits : NULL);

		/*
		 * In the case we modified a previously modified tuple again, free
		 * the memory from the previous run
		 */
		if ((char *) olddata != ((char *) newtup + HEAPTUPLESIZE))
			pfree(olddata);

		/*
		 * Switch back to the old memory context
		 */
		MemoryContextSwitchTo(oldcxt);
	}

	/*
	 * Free allocated temp values
	 */
	if (need_free)
		for (i = 0; i < numAttrs; i++)
			if (toast_free[i])
				pfree(DatumGetPointer(toast_values[i]));

	/*
	 * Delete external values from the old tuple
	 */
	if (need_delold)
		for (i = 0; i < numAttrs; i++)
			if (toast_delold[i])
				toast_delete_datum(rel,
					heap_getattr(oldtup, i + 1, tupleDesc, &old_isnull));
}


/* ----------
 * toast_compress_datum -
 *
 *	Create a compressed version of a varlena datum
 *
 *	If we fail (ie, compressed result is actually bigger than original)
 *	then return NULL.  We must not use compressed data if it'd expand
 *	the tuple!
 * ----------
 */
Datum
toast_compress_datum(Datum value)
{
	varattrib  *tmp;

	tmp = (varattrib *) palloc(sizeof(PGLZ_Header) + VARATT_SIZE(value));
	pglz_compress(VARATT_DATA(value), VARATT_SIZE(value) - VARHDRSZ,
				  (PGLZ_Header *) tmp,
				  PGLZ_strategy_default);
	if (VARATT_SIZE(tmp) < VARATT_SIZE(value))
	{
		/* successful compression */
		VARATT_SIZEP(tmp) |= VARATT_FLAG_COMPRESSED;
		return PointerGetDatum(tmp);
	}
	else
	{
		/* incompressible data */
		pfree(tmp);
		return PointerGetDatum(NULL);
	}
}


/* ----------
 * toast_save_datum -
 *
 *	Save one single datum into the secondary relation and return
 *	a varattrib reference for it.
 * ----------
 */
static Datum
toast_save_datum(Relation rel, Datum value)
{
	Relation	toastrel;
	Relation	toastidx;
	HeapTuple	toasttup;
	InsertIndexResult idxres;
	TupleDesc	toasttupDesc;
	Datum		t_values[3];
	char		t_nulls[3];
	varattrib  *result;
	struct
	{
		struct varlena hdr;
		char		data[TOAST_MAX_CHUNK_SIZE];
	}			chunk_data;
	int32		chunk_size;
	int32		chunk_seq = 0;
	char	   *data_p;
	int32		data_todo;

	/*
	 * Create the varattrib reference
	 */
	result = (varattrib *) palloc(sizeof(varattrib));

	result->va_header = sizeof(varattrib) | VARATT_FLAG_EXTERNAL;
	if (VARATT_IS_COMPRESSED(value))
	{
		result->va_header |= VARATT_FLAG_COMPRESSED;
		result->va_content.va_external.va_rawsize =
			((varattrib *) value)->va_content.va_compressed.va_rawsize;
	}
	else
		result->va_content.va_external.va_rawsize = VARATT_SIZE(value);

	result->va_content.va_external.va_extsize =
		VARATT_SIZE(value) - VARHDRSZ;
	result->va_content.va_external.va_valueid = newoid();
	result->va_content.va_external.va_toastrelid =
		rel->rd_rel->reltoastrelid;

	/*
	 * Initialize constant parts of the tuple data
	 */
	t_values[0] = ObjectIdGetDatum(result->va_content.va_external.va_valueid);
	t_values[2] = PointerGetDatum(&chunk_data);
	t_nulls[0] = ' ';
	t_nulls[1] = ' ';
	t_nulls[2] = ' ';

	/*
	 * Get the data to process
	 */
	data_p = VARATT_DATA(value);
	data_todo = VARATT_SIZE(value) - VARHDRSZ;

	/*
	 * Open the toast relation
	 */
	toastrel = heap_open(rel->rd_rel->reltoastrelid, RowExclusiveLock);
	toasttupDesc = toastrel->rd_att;
	toastidx = index_open(toastrel->rd_rel->reltoastidxid);

	/*
	 * Split up the item into chunks
	 */
	while (data_todo > 0)
	{
		/*
		 * Calculate the size of this chunk
		 */
		chunk_size = Min(TOAST_MAX_CHUNK_SIZE, data_todo);

		/*
		 * Build a tuple and store it
		 */
		t_values[1] = Int32GetDatum(chunk_seq++);
		VARATT_SIZEP(&chunk_data) = chunk_size + VARHDRSZ;
		memcpy(VARATT_DATA(&chunk_data), data_p, chunk_size);
		toasttup = heap_formtuple(toasttupDesc, t_values, t_nulls);
		if (!HeapTupleIsValid(toasttup))
			elog(ERROR, "Failed to build TOAST tuple");

		heap_insert(toastrel, toasttup);

		/*
		 * Create the index entry.	We cheat a little here by not using
		 * FormIndexDatum: this relies on the knowledge that the index
		 * columns are the same as the initial columns of the table.
		 *
		 * Note also that there had better not be any user-created index on
		 * the TOAST table, since we don't bother to update anything else.
		 */
		idxres = index_insert(toastidx, t_values, t_nulls,
							  &(toasttup->t_self),
							  toastrel);
		if (idxres == NULL)
			elog(ERROR, "Failed to insert index entry for TOAST tuple");

		/*
		 * Free memory
		 */
		pfree(idxres);
		heap_freetuple(toasttup);

		/*
		 * Move on to next chunk
		 */
		data_todo -= chunk_size;
		data_p += chunk_size;
	}

	/*
	 * Done - close toast relation and return the reference
	 */
	index_close(toastidx);
	heap_close(toastrel, RowExclusiveLock);

	return PointerGetDatum(result);
}


/* ----------
 * toast_delete_datum -
 *
 *	Delete a single external stored value.
 * ----------
 */
static void
toast_delete_datum(Relation rel, Datum value)
{
	register varattrib *attr = (varattrib *) value;
	Relation	toastrel;
	Relation	toastidx;
	ScanKeyData toastkey;
	IndexScanDesc toastscan;
	HeapTupleData toasttup;
	RetrieveIndexResult indexRes;
	Buffer		buffer;

	if (!VARATT_IS_EXTERNAL(attr))
		return;

	/*
	 * Open the toast relation and it's index
	 */
	toastrel = heap_open(attr->va_content.va_external.va_toastrelid,
						 RowExclusiveLock);
	toastidx = index_open(toastrel->rd_rel->reltoastidxid);

	/*
	 * Setup a scan key to fetch from the index by va_valueid (we don't
	 * particularly care whether we see them in sequence or not)
	 */
	ScanKeyEntryInitialize(&toastkey,
						   (bits16) 0,
						   (AttrNumber) 1,
						   (RegProcedure) F_OIDEQ,
			  ObjectIdGetDatum(attr->va_content.va_external.va_valueid));

	/*
	 * Find the chunks by index
	 */
	toastscan = index_beginscan(toastidx, false, 1, &toastkey);
	while ((indexRes = index_getnext(toastscan, ForwardScanDirection)) != NULL)
	{
		toasttup.t_self = indexRes->heap_iptr;
		heap_fetch(toastrel, SnapshotAny, &toasttup, &buffer, toastscan);
		pfree(indexRes);

		if (!toasttup.t_data)
			continue;

		/*
		 * Have a chunk, delete it
		 */
		simple_heap_delete(toastrel, &toasttup.t_self);

		ReleaseBuffer(buffer);
	}

	/*
	 * End scan and close relations
	 */
	index_endscan(toastscan);
	index_close(toastidx);
	heap_close(toastrel, RowExclusiveLock);
}


/* ----------
 * toast_fetch_datum -
 *
 *	Reconstruct an in memory varattrib from the chunks saved
 *	in the toast relation
 * ----------
 */
static varattrib *
toast_fetch_datum(varattrib *attr)
{
	Relation	toastrel;
	Relation	toastidx;
	ScanKeyData toastkey;
	IndexScanDesc toastscan;
	HeapTupleData toasttup;
	HeapTuple	ttup;
	TupleDesc	toasttupDesc;
	RetrieveIndexResult indexRes;
	Buffer		buffer;

	varattrib  *result;
	int32		ressize;
	int32		residx;
	int			numchunks;
	Pointer		chunk;
	bool		isnull;
	int32		chunksize;

	char	   *chunks_found;
	char	   *chunks_expected;

	ressize = attr->va_content.va_external.va_extsize;
	numchunks = ((ressize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;

	chunks_found = palloc(numchunks);
	chunks_expected = palloc(numchunks);
	memset(chunks_found, 0, numchunks);
	memset(chunks_expected, 1, numchunks);

	result = (varattrib *) palloc(ressize + VARHDRSZ);
	VARATT_SIZEP(result) = ressize + VARHDRSZ;
	if (VARATT_IS_COMPRESSED(attr))
		VARATT_SIZEP(result) |= VARATT_FLAG_COMPRESSED;

	/*
	 * Open the toast relation and it's index
	 */
	toastrel = heap_open(attr->va_content.va_external.va_toastrelid,
						 AccessShareLock);
	toasttupDesc = toastrel->rd_att;
	toastidx = index_open(toastrel->rd_rel->reltoastidxid);

	/*
	 * Setup a scan key to fetch from the index by va_valueid
	 */
	ScanKeyEntryInitialize(&toastkey,
						   (bits16) 0,
						   (AttrNumber) 1,
						   (RegProcedure) F_OIDEQ,
			  ObjectIdGetDatum(attr->va_content.va_external.va_valueid));

	/*
	 * Read the chunks by index
	 *
	 * Note we will not necessarily see the chunks in sequence-number order.
	 */
	toastscan = index_beginscan(toastidx, false, 1, &toastkey);
	while ((indexRes = index_getnext(toastscan, ForwardScanDirection)) != NULL)
	{
		toasttup.t_self = indexRes->heap_iptr;
		heap_fetch(toastrel, SnapshotAny, &toasttup, &buffer, toastscan);
		pfree(indexRes);

		if (toasttup.t_data == NULL)
			continue;
		ttup = &toasttup;

		/*
		 * Have a chunk, extract the sequence number and the data
		 */
		residx = DatumGetInt32(heap_getattr(ttup, 2, toasttupDesc, &isnull));
		Assert(!isnull);
		chunk = DatumGetPointer(heap_getattr(ttup, 3, toasttupDesc, &isnull));
		Assert(!isnull);
		chunksize = VARATT_SIZE(chunk) - VARHDRSZ;

		/*
		 * Some checks on the data we've found
		 */
		if (residx < 0 || residx >= numchunks)
			elog(ERROR, "unexpected chunk number %d for toast value %u",
				 residx,
				 attr->va_content.va_external.va_valueid);
		if (residx < numchunks - 1)
		{
			if (chunksize != TOAST_MAX_CHUNK_SIZE)
				elog(ERROR, "unexpected chunk size %d in chunk %d for toast value %u",
					 chunksize, residx,
					 attr->va_content.va_external.va_valueid);
		}
		else
		{
			if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != ressize)
				elog(ERROR, "unexpected chunk size %d in chunk %d for toast value %u",
					 chunksize, residx,
					 attr->va_content.va_external.va_valueid);
		}
		if (chunks_found[residx]++ > 0)
			elog(ERROR, "chunk %d for toast value %u appears multiple times",
				 residx,
				 attr->va_content.va_external.va_valueid);

		/*
		 * Copy the data into proper place in our result
		 */
		memcpy(((char *) VARATT_DATA(result)) + residx * TOAST_MAX_CHUNK_SIZE,
			   VARATT_DATA(chunk),
			   chunksize);

		ReleaseBuffer(buffer);
	}

	/*
	 * Final checks that we successfully fetched the datum
	 */
	if (memcmp(chunks_found, chunks_expected, numchunks) != 0)
		elog(ERROR, "not all toast chunks found for value %u",
			 attr->va_content.va_external.va_valueid);
	pfree(chunks_expected);
	pfree(chunks_found);

	/*
	 * End scan and close relations
	 */
	index_endscan(toastscan);
	index_close(toastidx);
	heap_close(toastrel, AccessShareLock);

	return result;
}

#endif	 /* TUPLE_TOASTER_ACTIVE */