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/* Copyright (C) 2000-2002 MySQL AB
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Copyright (C) 2008 eBay, Inc
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; version 2 of the License.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
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/* Implements various base dataspace-related functions - allocate, free, clear */
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MySQL Heap tables keep data in arrays of fixed-size chunks.
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These chunks are organized into two groups of HP_BLOCK structures:
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- group1 contains indexes, with one HP_BLOCK per key
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- group2 contains record data, with single HP_BLOCK
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for all records, referenced by HP_SHARE.recordspace.block
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While columns used in index are usually small, other columns
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in the table may need to accomodate larger data. Typically,
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larger data is placed into VARCHAR or BLOB columns. With actual
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sizes varying, Heap Engine has to support variable-sized records
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in memory. Heap Engine implements the concept of dataspace
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(HP_DATASPACE), which incorporates HP_BLOCK for the record data,
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and adds more information for managing variable-sized records.
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Variable-size records are stored in multiple "chunks",
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which means that a single record of data (database "row") can
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consist of multiple chunks organized into one "set". HP_BLOCK
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contains chunks. In variable-size format, one record
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is represented as one or many chunks, depending on the actual
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data, while in fixed-size mode, one record is always represented
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as one chunk. The index structures would always point to the first
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chunk in the chunkset.
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At the time of table creation, Heap Engine attempts to find out
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if variable-size records are desired. A user can request
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variable-size records by providing either row_type=dynamic or
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block_size=NNN table create option. Heap Engine will check
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whether block_size provides enough space in the first chunk
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to keep all null bits and columns that are used in indexes.
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If block_size is too small, table creation will be aborted
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with an error. Heap Engine will revert to fixed-size allocation
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mode if block_size provides no memory benefits (no VARCHAR
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fields extending past first chunk).
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In order to improve index search performance, Heap Engine needs
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to keep all null flags and all columns used as keys inside
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the first chunk of a chunkset. In particular, this means that
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all columns used as keys should be defined first in the table
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creation SQL. The length of data used by null bits and key columns
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is stored as fixed_data_length inside HP_SHARE. fixed_data_length
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will extend past last key column if more fixed-length fields can
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fit into the first chunk.
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Variable-size records are necessary only in the presence
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of variable-size columns. Heap Engine will be looking for VARCHAR
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columns, which declare length of 32 or more. If no such columns
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are found, table will be switched to fixed-size format. You should
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always try to put such columns at the end of the table definition.
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Whenever data is being inserted or updated in the table
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Heap Engine will calculate how many chunks are necessary.
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For insert operations, Heap Engine allocates new chunkset in
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the recordspace. For update operations it will modify length of
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the existing chunkset, unlinking unnecessary chunks at the end,
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or allocating and adding more if larger length is necessary.
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When writing data to chunks or copying data back to record,
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Heap Engine will first copy fixed_data_length of data using single
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memcpy call. The rest of the columns are processed one-by-one.
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Non-VARCHAR columns are copied in their full format. VARCHAR's
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are copied based on their actual length. Any NULL values after
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fixed_data_length are skipped.
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The allocation and contents of the actual chunks varies between
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fixed and variable-size modes. Total chunk length is always
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aligned to the next sizeof(unsigned char*). Here is the format of
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unsigned char[] - sizeof=chunk_dataspace_length, but at least
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sizeof(unsigned char*) bytes. Keeps actual data or pointer
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to the next deleted chunk.
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chunk_dataspace_length equals to full record length
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unsigned char - status field (1 means "in use", 0 means "deleted")
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Variable-size uses different format:
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unsigned char[] - sizeof=chunk_dataspace_length, but at least
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sizeof(unsigned char*) bytes. Keeps actual data or pointer
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to the next deleted chunk.
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chunk_dataspace_length is set according to table
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unsigned char* - pointer to the next chunk in this chunkset,
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or NULL for the last chunk
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unsigned char - status field (1 means "first", 0 means "deleted",
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When allocating a new chunkset of N chunks, Heap Engine will try
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to allocate chunks one-by-one, linking them as they become
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allocated. Allocation of a single chunk will attempt to reuse
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a deleted (freed) chunk. If no free chunks are available,
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it will attempt to allocate a new area inside HP_BLOCK.
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Freeing chunks will place them at the front of free list
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referenced by del_link in HP_DATASPACE. The newly freed chunk
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will contain reference to the previously freed chunk in its first
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sizeof(unsigned char*) of the payload space.
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1. It is not very nice to require people to keep key columns
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at the beginning of the table creation SQL. There are three
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proposed resolutions:
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a. Leave it as is. It's a reasonable limitation
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b. Add new HA_KEEP_KEY_COLUMNS_TO_FRONT flag to handler.h and
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make table.cpp align columns when it creates the table
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c. Make HeapEngine reorder columns in the chunk data, so that
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key columns go first. Add parallel HA_KEYSEG structures
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to distinguish positions in record vs. positions in
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the first chunk. Copy all data field-by-field rather than
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using single memcpy unless DBA kept key columns to
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2. heap_check_heap needs verify linked chunks, looking for
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issues such as orphans, cycles, and bad links. However,
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Heap Engine today does not do similar things even for
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3. With new HP_DATASPACE allocation mechaism, BLOB will become
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increasingly simple to implement, but I may not have time
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for that. In one approach, BLOB data can be placed at
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the end of the same record. In another approach (which I
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prefer) BLOB data would have its own HP_DATASPACE with
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variable-size entries.
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4. In a more sophisticated implementation, some space can
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be saved even with all fixed-size columns if many of them
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have NULL value, as long as these columns are not used
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5. In variable-size format status should be moved to lower
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bits of the "next" pointer. Pointer is always aligned
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to sizeof(unsigned char*), which is at least 4, leaving 2 lower
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bits free. This will save 8 bytes per chunk
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6. As we do not want to modify FRM format, BLOCK_SIZE option
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of "CREATE TABLE" is saved as "RAID_CHUNKSIZE" for
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static unsigned char *hp_allocate_one_chunk(HP_DATASPACE *info);
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Frees memory and zeros-out any relevant counters in the dataspace
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@param info the dataspace to clear
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void hp_clear_dataspace(HP_DATASPACE *info)
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if (info->block.levels)
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hp_free_level(&info->block,info->block.levels,info->block.root,
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info->block.levels=0;
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info->del_chunk_count= info->chunk_count= 0;
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info->total_data_length= 0;
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Allocate or reallocate a chunkset in the dataspace
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Attempts to allocate a new chunkset or change the size of an existing chunkset
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@param info the hosting dataspace
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@param chunk_count the number of chunks that we expect as the result
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@param existing_set non-null value asks function to resize existing chunkset,
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return value would point to this set
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@return Pointer to the first chunk in the new or updated chunkset, or NULL if unsuccessful
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static unsigned char *hp_allocate_variable_chunkset(HP_DATASPACE *info,
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uint32_t chunk_count, unsigned char* existing_set)
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int alloc_count= chunk_count, i;
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unsigned char *first_chunk= 0, *curr_chunk= 0, *prev_chunk= 0, *last_existing_chunk= 0;
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first_chunk= existing_set;
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curr_chunk= existing_set;
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while (curr_chunk && alloc_count)
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prev_chunk= curr_chunk;
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curr_chunk= *((unsigned char**)(curr_chunk + info->offset_link));
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/* We came through all chunks and there is more left, let's truncate the list */
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*((unsigned char**)(prev_chunk + info->offset_link)) = NULL;
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hp_free_chunks(info, curr_chunk);
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last_existing_chunk = prev_chunk;
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/* We can reach this point only if we're allocating new chunkset or more chunks in existing set */
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for (i=0; i<alloc_count; i++)
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curr_chunk= hp_allocate_one_chunk(info);
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/* no space in the current block */
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if (last_existing_chunk)
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/* Truncate whatever was added at the end of the existing chunkset */
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prev_chunk= last_existing_chunk;
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curr_chunk= *((unsigned char**)(prev_chunk + info->offset_link));
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*((unsigned char**)(prev_chunk + info->offset_link)) = NULL;
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hp_free_chunks(info, curr_chunk);
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else if (first_chunk)
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/* free any chunks previously allocated */
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hp_free_chunks(info, first_chunk);
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/* mark as if this chunk is last in the chunkset */
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*((unsigned char**) (curr_chunk + info->offset_link))= 0;
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/* tie them into a linked list */
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*((unsigned char**) (prev_chunk + info->offset_link))= curr_chunk;
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curr_chunk[info->offset_status]= CHUNK_STATUS_LINKED; /* Record linked from active */
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curr_chunk[info->offset_status]= CHUNK_STATUS_ACTIVE; /* Record active */
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first_chunk= curr_chunk;
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prev_chunk= curr_chunk;
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Allocate a new chunkset in the dataspace
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Attempts to allocate a new chunkset
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@param info the hosting dataspace
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@param chunk_count the number of chunks that we expect as the result
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@return Pointer to the first chunk in the new or updated chunkset, or NULL if unsuccessful
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unsigned char *hp_allocate_chunkset(HP_DATASPACE *info, uint32_t chunk_count)
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unsigned char* result;
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if (info->is_variable_size)
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result = hp_allocate_variable_chunkset(info, chunk_count, NULL);
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result= hp_allocate_one_chunk(info);
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result[info->offset_status]= CHUNK_STATUS_ACTIVE;
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Reallocate an existing chunkset in the dataspace
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Attempts to change the size of an existing chunkset
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@param info the hosting dataspace
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@param chunk_count the number of chunks that we expect as the result
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@param pos pointer to the existing chunkset
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@return Error code or zero if successful
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int hp_reallocate_chunkset(HP_DATASPACE *info, uint32_t chunk_count, unsigned char* pos)
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if (!info->is_variable_size)
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/* Update should never change chunk_count in fixed-size mode */
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my_errno=HA_ERR_WRONG_COMMAND;
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/* Reallocate never moves the first chunk */
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if (!hp_allocate_variable_chunkset(info, chunk_count, pos))
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Allocate a single chunk in the dataspace
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Attempts to allocate a new chunk or reuse one from deleted list
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@param info the hosting dataspace
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@return Pointer to the chunk, or NULL if unsuccessful
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static unsigned char *hp_allocate_one_chunk(HP_DATASPACE *info)
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unsigned char* curr_chunk;
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size_t length, block_pos;
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curr_chunk=info->del_link;
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info->del_link= *((unsigned char**) curr_chunk);
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info->del_chunk_count--;
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block_pos= (info->chunk_count % info->block.records_in_block);
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if (hp_get_new_block(&info->block,&length))
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/* no space in the current block */
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info->total_data_length+= length;
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curr_chunk= ((unsigned char*) info->block.level_info[0].last_blocks +
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block_pos * info->block.recbuffer);
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Free a list of chunks
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Reclaims all chunks linked by the pointer,
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which could be the whole chunkset or a part of an existing chunkset
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@param info the hosting dataspace
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@param pos pointer to the head of the chunkset
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void hp_free_chunks(HP_DATASPACE *info, unsigned char *pos)
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unsigned char* curr_chunk= pos;
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info->del_chunk_count++;
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*((unsigned char**) curr_chunk)= info->del_link;
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info->del_link= curr_chunk;
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curr_chunk[info->offset_status]= CHUNK_STATUS_DELETED;
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if (!info->is_variable_size)
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/* Delete next chunk in this chunkset */
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curr_chunk= *((unsigned char**)(curr_chunk + info->offset_link));
added
removed
storage/heap/hp_dspace.c
