src/backend/access/common/detoast.c

/*-------------------------------------------------------------------------
 *
 * detoast.c
 *	  Retrieve compressed or external variable size attributes.
 *
 * Copyright (c) 2000-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  src/backend/access/common/detoast.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/detoast.h"
#include "access/table.h"
#include "access/tableam.h"
#include "access/toast_internals.h"
#include "common/int.h"
#include "common/pg_lzcompress.h"
#include "utils/expandeddatum.h"
#include "utils/rel.h"

static struct varlena *toast_fetch_datum(struct varlena *attr);
static struct varlena *toast_fetch_datum_slice(struct varlena *attr,
											   int32 sliceoffset,
											   int32 slicelength);
static struct varlena *toast_decompress_datum(struct varlena *attr);
static struct varlena *toast_decompress_datum_slice(struct varlena *attr, int32 slicelength);

/* ----------
 * detoast_external_attr -
 *
 *	Public entry point to get back a toasted value from
 *	external source (possibly still in compressed format).
 *
 * This will return a datum that contains all the data internally, ie, not
 * relying on external storage or memory, but it can still be compressed or
 * have a short header.  Note some callers assume that if the input is an
 * EXTERNAL datum, the result will be a pfree'able chunk.
 * ----------
 */
struct varlena *
detoast_external_attr(struct varlena *attr)
{
	struct varlena *result;

	if (VARATT_IS_EXTERNAL_ONDISK(attr))
	{
		/*
		 * This is an external stored plain value
		 */
		result = toast_fetch_datum(attr);
	}
	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
	{
		/*
		 * This is an indirect pointer --- dereference it
		 */
		struct varatt_indirect redirect;

		VARATT_EXTERNAL_GET_POINTER(redirect, attr);
		attr = (struct varlena *) redirect.pointer;

		/* nested indirect Datums aren't allowed */
		Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));

		/* recurse if value is still external in some other way */
		if (VARATT_IS_EXTERNAL(attr))
			return detoast_external_attr(attr);

		/*
		 * Copy into the caller's memory context, in case caller tries to
		 * pfree the result.
		 */
		result = (struct varlena *) palloc(VARSIZE_ANY(attr));
		memcpy(result, attr, VARSIZE_ANY(attr));
	}
	else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
	{
		/*
		 * This is an expanded-object pointer --- get flat format
		 */
		ExpandedObjectHeader *eoh;
		Size		resultsize;

		eoh = DatumGetEOHP(PointerGetDatum(attr));
		resultsize = EOH_get_flat_size(eoh);
		result = (struct varlena *) palloc(resultsize);
		EOH_flatten_into(eoh, (void *) result, resultsize);
	}
	else
	{
		/*
		 * This is a plain value inside of the main tuple - why am I called?
		 */
		result = attr;
	}

	return result;
}


/* ----------
 * detoast_attr -
 *
 *	Public entry point to get back a toasted value from compression
 *	or external storage.  The result is always non-extended varlena form.
 *
 * Note some callers assume that if the input is an EXTERNAL or COMPRESSED
 * datum, the result will be a pfree'able chunk.
 * ----------
 */
struct varlena *
detoast_attr(struct varlena *attr)
{
	if (VARATT_IS_EXTERNAL_ONDISK(attr))
	{
		/*
		 * This is an externally stored datum --- fetch it back from there
		 */
		attr = toast_fetch_datum(attr);
		/* If it's compressed, decompress it */
		if (VARATT_IS_COMPRESSED(attr))
		{
			struct varlena *tmp = attr;

			attr = toast_decompress_datum(tmp);
			pfree(tmp);
		}
	}
	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
	{
		/*
		 * This is an indirect pointer --- dereference it
		 */
		struct varatt_indirect redirect;

		VARATT_EXTERNAL_GET_POINTER(redirect, attr);
		attr = (struct varlena *) redirect.pointer;

		/* nested indirect Datums aren't allowed */
		Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));

		/* recurse in case value is still extended in some other way */
		attr = detoast_attr(attr);

		/* if it isn't, we'd better copy it */
		if (attr == (struct varlena *) redirect.pointer)
		{
			struct varlena *result;

			result = (struct varlena *) palloc(VARSIZE_ANY(attr));
			memcpy(result, attr, VARSIZE_ANY(attr));
			attr = result;
		}
	}
	else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
	{
		/*
		 * This is an expanded-object pointer --- get flat format
		 */
		attr = detoast_external_attr(attr);
		/* flatteners are not allowed to produce compressed/short output */
		Assert(!VARATT_IS_EXTENDED(attr));
	}
	else if (VARATT_IS_COMPRESSED(attr))
	{
		/*
		 * This is a compressed value inside of the main tuple
		 */
		attr = toast_decompress_datum(attr);
	}
	else if (VARATT_IS_SHORT(attr))
	{
		/*
		 * This is a short-header varlena --- convert to 4-byte header format
		 */
		Size		data_size = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT;
		Size		new_size = data_size + VARHDRSZ;
		struct varlena *new_attr;

		new_attr = (struct varlena *) palloc(new_size);
		SET_VARSIZE(new_attr, new_size);
		memcpy(VARDATA(new_attr), VARDATA_SHORT(attr), data_size);
		attr = new_attr;
	}

	return attr;
}


/* ----------
 * detoast_attr_slice -
 *
 *		Public entry point to get back part of a toasted value
 *		from compression or external storage.
 *
 * sliceoffset is where to start (zero or more)
 * If slicelength < 0, return everything beyond sliceoffset
 * ----------
 */
struct varlena *
detoast_attr_slice(struct varlena *attr,
				   int32 sliceoffset, int32 slicelength)
{
	struct varlena *preslice;
	struct varlena *result;
	char	   *attrdata;
	int32		slicelimit;
	int32		attrsize;

	if (sliceoffset < 0)
		elog(ERROR, "invalid sliceoffset: %d", sliceoffset);

	/*
	 * Compute slicelimit = offset + length, or -1 if we must fetch all of the
	 * value.  In case of integer overflow, we must fetch all.
	 */
	if (slicelength < 0)
		slicelimit = -1;
	else if (pg_add_s32_overflow(sliceoffset, slicelength, &slicelimit))
		slicelength = slicelimit = -1;

	if (VARATT_IS_EXTERNAL_ONDISK(attr))
	{
		struct varatt_external toast_pointer;

		VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

		/* fast path for non-compressed external datums */
		if (!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
			return toast_fetch_datum_slice(attr, sliceoffset, slicelength);

		/*
		 * For compressed values, we need to fetch enough slices to decompress
		 * at least the requested part (when a prefix is requested).
		 * Otherwise, just fetch all slices.
		 */
		if (slicelimit >= 0)
		{
			int32		max_size = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);

			/*
			 * Determine maximum amount of compressed data needed for a prefix
			 * of a given length (after decompression).
			 *
			 * At least for now, if it's LZ4 data, we'll have to fetch the
			 * whole thing, because there doesn't seem to be an API call to
			 * determine how much compressed data we need to be sure of being
			 * able to decompress the required slice.
			 */
			if (VARATT_EXTERNAL_GET_COMPRESS_METHOD(toast_pointer) ==
				TOAST_PGLZ_COMPRESSION_ID)
				max_size = pglz_maximum_compressed_size(slicelimit, max_size);

			/*
			 * Fetch enough compressed slices (compressed marker will get set
			 * automatically).
			 */
			preslice = toast_fetch_datum_slice(attr, 0, max_size);
		}
		else
			preslice = toast_fetch_datum(attr);
	}
	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
	{
		struct varatt_indirect redirect;

		VARATT_EXTERNAL_GET_POINTER(redirect, attr);

		/* nested indirect Datums aren't allowed */
		Assert(!VARATT_IS_EXTERNAL_INDIRECT(redirect.pointer));

		return detoast_attr_slice(redirect.pointer,
								  sliceoffset, slicelength);
	}
	else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
	{
		/* pass it off to detoast_external_attr to flatten */
		preslice = detoast_external_attr(attr);
	}
	else
		preslice = attr;

	Assert(!VARATT_IS_EXTERNAL(preslice));

	if (VARATT_IS_COMPRESSED(preslice))
	{
		struct varlena *tmp = preslice;

		/* Decompress enough to encompass the slice and the offset */
		if (slicelimit >= 0)
			preslice = toast_decompress_datum_slice(tmp, slicelimit);
		else
			preslice = toast_decompress_datum(tmp);

		if (tmp != attr)
			pfree(tmp);
	}

	if (VARATT_IS_SHORT(preslice))
	{
		attrdata = VARDATA_SHORT(preslice);
		attrsize = VARSIZE_SHORT(preslice) - VARHDRSZ_SHORT;
	}
	else
	{
		attrdata = VARDATA(preslice);
		attrsize = VARSIZE(preslice) - VARHDRSZ;
	}

	/* slicing of datum for compressed cases and plain value */

	if (sliceoffset >= attrsize)
	{
		sliceoffset = 0;
		slicelength = 0;
	}
	else if (slicelength < 0 || slicelimit > attrsize)
		slicelength = attrsize - sliceoffset;

	result = (struct varlena *) palloc(slicelength + VARHDRSZ);
	SET_VARSIZE(result, slicelength + VARHDRSZ);

	memcpy(VARDATA(result), attrdata + sliceoffset, slicelength);

	if (preslice != attr)
		pfree(preslice);

	return result;
}

/* ----------
 * toast_fetch_datum -
 *
 *	Reconstruct an in memory Datum from the chunks saved
 *	in the toast relation
 * ----------
 */
static struct varlena *
toast_fetch_datum(struct varlena *attr)
{
	Relation	toastrel;
	struct varlena *result;
	struct varatt_external toast_pointer;
	int32		attrsize;

	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
		elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");

	/* Must copy to access aligned fields */
	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

	attrsize = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);

	result = (struct varlena *) palloc(attrsize + VARHDRSZ);

	if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
		SET_VARSIZE_COMPRESSED(result, attrsize + VARHDRSZ);
	else
		SET_VARSIZE(result, attrsize + VARHDRSZ);

	if (attrsize == 0)
		return result;			/* Probably shouldn't happen, but just in
								 * case. */

	/*
	 * Open the toast relation and its indexes
	 */
	toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);

	/* Fetch all chunks */
	table_relation_fetch_toast_slice(toastrel, toast_pointer.va_valueid,
									 attrsize, 0, attrsize, result);

	/* Close toast table */
	table_close(toastrel, AccessShareLock);

	return result;
}

/* ----------
 * toast_fetch_datum_slice -
 *
 *	Reconstruct a segment of a Datum from the chunks saved
 *	in the toast relation
 *
 *	Note that this function supports non-compressed external datums
 *	and compressed external datums (in which case the requested slice
 *	has to be a prefix, i.e. sliceoffset has to be 0).
 * ----------
 */
static struct varlena *
toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
						int32 slicelength)
{
	Relation	toastrel;
	struct varlena *result;
	struct varatt_external toast_pointer;
	int32		attrsize;

	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
		elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");

	/* Must copy to access aligned fields */
	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

	/*
	 * It's nonsense to fetch slices of a compressed datum unless when it's a
	 * prefix -- this isn't lo_* we can't return a compressed datum which is
	 * meaningful to toast later.
	 */
	Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) || 0 == sliceoffset);

	attrsize = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);

	if (sliceoffset >= attrsize)
	{
		sliceoffset = 0;
		slicelength = 0;
	}

	/*
	 * When fetching a prefix of a compressed external datum, account for the
	 * space required by va_tcinfo, which is stored at the beginning as an
	 * int32 value.
	 */
	if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) && slicelength > 0)
		slicelength = slicelength + sizeof(int32);

	/*
	 * Adjust length request if needed.  (Note: our sole caller,
	 * detoast_attr_slice, protects us against sliceoffset + slicelength
	 * overflowing.)
	 */
	if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
		slicelength = attrsize - sliceoffset;

	result = (struct varlena *) palloc(slicelength + VARHDRSZ);

	if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
		SET_VARSIZE_COMPRESSED(result, slicelength + VARHDRSZ);
	else
		SET_VARSIZE(result, slicelength + VARHDRSZ);

	if (slicelength == 0)
		return result;			/* Can save a lot of work at this point! */

	/* Open the toast relation */
	toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);

	/* Fetch all chunks */
	table_relation_fetch_toast_slice(toastrel, toast_pointer.va_valueid,
									 attrsize, sliceoffset, slicelength,
									 result);

	/* Close toast table */
	table_close(toastrel, AccessShareLock);

	return result;
}

/* ----------
 * toast_decompress_datum -
 *
 * Decompress a compressed version of a varlena datum
 */
static struct varlena *
toast_decompress_datum(struct varlena *attr)
{
	ToastCompressionId cmid;

	Assert(VARATT_IS_COMPRESSED(attr));

	/*
	 * Fetch the compression method id stored in the compression header and
	 * decompress the data using the appropriate decompression routine.
	 */
	cmid = TOAST_COMPRESS_METHOD(attr);
	switch (cmid)
	{
		case TOAST_PGLZ_COMPRESSION_ID:
			return pglz_decompress_datum(attr);
		case TOAST_LZ4_COMPRESSION_ID:
			return lz4_decompress_datum(attr);
		default:
			elog(ERROR, "invalid compression method id %d", cmid);
			return NULL;		/* keep compiler quiet */
	}
}


/* ----------
 * toast_decompress_datum_slice -
 *
 * Decompress the front of a compressed version of a varlena datum.
 * offset handling happens in detoast_attr_slice.
 * Here we just decompress a slice from the front.
 */
static struct varlena *
toast_decompress_datum_slice(struct varlena *attr, int32 slicelength)
{
	ToastCompressionId cmid;

	Assert(VARATT_IS_COMPRESSED(attr));

	/*
	 * Some callers may pass a slicelength that's more than the actual
	 * decompressed size.  If so, just decompress normally.  This avoids
	 * possibly allocating a larger-than-necessary result object, and may be
	 * faster and/or more robust as well.  Notably, some versions of liblz4
	 * have been seen to give wrong results if passed an output size that is
	 * more than the data's true decompressed size.
	 */
	if ((uint32) slicelength >= TOAST_COMPRESS_EXTSIZE(attr))
		return toast_decompress_datum(attr);

	/*
	 * Fetch the compression method id stored in the compression header and
	 * decompress the data slice using the appropriate decompression routine.
	 */
	cmid = TOAST_COMPRESS_METHOD(attr);
	switch (cmid)
	{
		case TOAST_PGLZ_COMPRESSION_ID:
			return pglz_decompress_datum_slice(attr, slicelength);
		case TOAST_LZ4_COMPRESSION_ID:
			return lz4_decompress_datum_slice(attr, slicelength);
		default:
			elog(ERROR, "invalid compression method id %d", cmid);
			return NULL;		/* keep compiler quiet */
	}
}

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

	if (VARATT_IS_EXTERNAL_ONDISK(attr))
	{
		/* va_rawsize is the size of the original datum -- including header */
		struct varatt_external toast_pointer;

		VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
		result = toast_pointer.va_rawsize;
	}
	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
	{
		struct varatt_indirect toast_pointer;

		VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

		/* nested indirect Datums aren't allowed */
		Assert(!VARATT_IS_EXTERNAL_INDIRECT(toast_pointer.pointer));

		return toast_raw_datum_size(PointerGetDatum(toast_pointer.pointer));
	}
	else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
	{
		result = EOH_get_flat_size(DatumGetEOHP(value));
	}
	else if (VARATT_IS_COMPRESSED(attr))
	{
		/* here, va_rawsize is just the payload size */
		result = VARDATA_COMPRESSED_GET_EXTSIZE(attr) + VARHDRSZ;
	}
	else if (VARATT_IS_SHORT(attr))
	{
		/*
		 * we have to normalize the header length to VARHDRSZ or else the
		 * callers of this function will be confused.
		 */
		result = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT + VARHDRSZ;
	}
	else
	{
		/* plain untoasted datum */
		result = VARSIZE(attr);
	}
	return result;
}

/* ----------
 * toast_datum_size
 *
 *	Return the physical storage size (possibly compressed) of a varlena datum
 * ----------
 */
Size
toast_datum_size(Datum value)
{
	struct varlena *attr = (struct varlena *) DatumGetPointer(value);
	Size		result;

	if (VARATT_IS_EXTERNAL_ONDISK(attr))
	{
		/*
		 * Attribute is stored externally - return the extsize whether
		 * compressed or not.  We do not count the size of the toast pointer
		 * ... should we?
		 */
		struct varatt_external toast_pointer;

		VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
		result = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);
	}
	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
	{
		struct varatt_indirect toast_pointer;

		VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

		/* nested indirect Datums aren't allowed */
		Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));

		return toast_datum_size(PointerGetDatum(toast_pointer.pointer));
	}
	else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
	{
		result = EOH_get_flat_size(DatumGetEOHP(value));
	}
	else if (VARATT_IS_SHORT(attr))
	{
		result = VARSIZE_SHORT(attr);
	}
	else
	{
		/*
		 * Attribute is stored inline either compressed or not, just calculate
		 * the size of the datum in either case.
		 */
		result = VARSIZE(attr);
	}
	return result;
}

/*
 * If this function is changed then update varattrib_untoast_len as well
 */
void
varattrib_untoast_ptr_len(Datum d, char **datastart, int *len, void **tofree)
{
	if (DatumGetPointer(d) == NULL)
	{
		ereport(ERROR,
				(errcode(ERRCODE_INTERNAL_ERROR),
				 errmsg(" Unable to detoast datum "),
				 errprintstack(true)));
	}

	struct varlena *va = (struct varlena *) DatumGetPointer(d);
	struct varlena *attr = va;

	*len = -1;
	*tofree = NULL;

	if (VARATT_IS_EXTENDED(va))
	{
		if (VARATT_IS_EXTERNAL(va))
		{
			attr = toast_fetch_datum(va);
			/* toast_fetch_datum will palloc, so set it up for free */
			*tofree = attr;
		}

		if (VARATT_IS_COMPRESSED(attr))
		{
			struct varlena *tmp = attr;
			attr = toast_decompress_datum(tmp);

			/* If tofree is set, that is, we get it from toast_fetch_datum.  
			 * We need to free it here 
			 */
			if(*tofree)
				pfree(*tofree);
			*tofree = attr;
		}
		else if (VARATT_IS_SHORT(attr))
		{
		    /* Warning! Return unaligned pointer! */
			*len = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT;
			*datastart = VARDATA_SHORT(attr);
			attr = NULL;
		}
	}

	if (*len == -1)
	{
		*datastart = VARDATA(attr);
		*len = VARSIZE(attr) - VARHDRSZ;
	}

	Assert(*len >= 0);
}