/* Copyright (C) 2000-2002 MySQL AB Copyright (C) 2008 eBay, Inc This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; version 2 of the License. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ /* Implements various base dataspace-related functions - allocate, free, clear */ #include "heapdef.h" /* MySQL Heap tables keep data in arrays of fixed-size chunks. These chunks are organized into two groups of HP_BLOCK structures: - group1 contains indexes, with one HP_BLOCK per key (part of HP_KEYDEF) - group2 contains record data, with single HP_BLOCK for all records, referenced by HP_SHARE.recordspace.block While columns used in index are usually small, other columns in the table may need to accomodate larger data. Typically, larger data is placed into VARCHAR or BLOB columns. With actual sizes varying, Heap Engine has to support variable-sized records in memory. Heap Engine implements the concept of dataspace (HP_DATASPACE), which incorporates HP_BLOCK for the record data, and adds more information for managing variable-sized records. Variable-size records are stored in multiple "chunks", which means that a single record of data (database "row") can consist of multiple chunks organized into one "set". HP_BLOCK contains chunks. In variable-size format, one record is represented as one or many chunks, depending on the actual data, while in fixed-size mode, one record is always represented as one chunk. The index structures would always point to the first chunk in the chunkset. At the time of table creation, Heap Engine attempts to find out if variable-size records are desired. A user can request variable-size records by providing either row_type=dynamic or block_size=NNN table create option. Heap Engine will check whether block_size provides enough space in the first chunk to keep all null bits and columns that are used in indexes. If block_size is too small, table creation will be aborted with an error. Heap Engine will revert to fixed-size allocation mode if block_size provides no memory benefits (no VARCHAR fields extending past first chunk). In order to improve index search performance, Heap Engine needs to keep all null flags and all columns used as keys inside the first chunk of a chunkset. In particular, this means that all columns used as keys should be defined first in the table creation SQL. The length of data used by null bits and key columns is stored as fixed_data_length inside HP_SHARE. fixed_data_length will extend past last key column if more fixed-length fields can fit into the first chunk. Variable-size records are necessary only in the presence of variable-size columns. Heap Engine will be looking for VARCHAR columns, which declare length of 32 or more. If no such columns are found, table will be switched to fixed-size format. You should always try to put such columns at the end of the table definition. Whenever data is being inserted or updated in the table Heap Engine will calculate how many chunks are necessary. For insert operations, Heap Engine allocates new chunkset in the recordspace. For update operations it will modify length of the existing chunkset, unlinking unnecessary chunks at the end, or allocating and adding more if larger length is necessary. When writing data to chunks or copying data back to record, Heap Engine will first copy fixed_data_length of data using single memcpy call. The rest of the columns are processed one-by-one. Non-VARCHAR columns are copied in their full format. VARCHAR's are copied based on their actual length. Any NULL values after fixed_data_length are skipped. The allocation and contents of the actual chunks varies between fixed and variable-size modes. Total chunk length is always aligned to the next sizeof(uchar*). Here is the format of fixed-size chunk: uchar[] - sizeof=chunk_dataspace_length, but at least sizeof(uchar*) bytes. Keeps actual data or pointer to the next deleted chunk. chunk_dataspace_length equals to full record length uchar - status field (1 means "in use", 0 means "deleted") Variable-size uses different format: uchar[] - sizeof=chunk_dataspace_length, but at least sizeof(uchar*) bytes. Keeps actual data or pointer to the next deleted chunk. chunk_dataspace_length is set according to table setup (block_size) uchar* - pointer to the next chunk in this chunkset, or NULL for the last chunk uchar - status field (1 means "first", 0 means "deleted", 2 means "linked") When allocating a new chunkset of N chunks, Heap Engine will try to allocate chunks one-by-one, linking them as they become allocated. Allocation of a single chunk will attempt to reuse a deleted (freed) chunk. If no free chunks are available, it will attempt to allocate a new area inside HP_BLOCK. Freeing chunks will place them at the front of free list referenced by del_link in HP_DATASPACE. The newly freed chunk will contain reference to the previously freed chunk in its first sizeof(uchar*) of the payload space. Here is open issues: 1. It is not very nice to require people to keep key columns at the beginning of the table creation SQL. There are three proposed resolutions: a. Leave it as is. It's a reasonable limitation b. Add new HA_KEEP_KEY_COLUMNS_TO_FRONT flag to handler.h and make table.cpp align columns when it creates the table c. Make HeapEngine reorder columns in the chunk data, so that key columns go first. Add parallel HA_KEYSEG structures to distinguish positions in record vs. positions in the first chunk. Copy all data field-by-field rather than using single memcpy unless DBA kept key columns to the beginning. 2. heap_check_heap needs verify linked chunks, looking for issues such as orphans, cycles, and bad links. However, Heap Engine today does not do similar things even for free list. 3. With new HP_DATASPACE allocation mechaism, BLOB will become increasingly simple to implement, but I may not have time for that. In one approach, BLOB data can be placed at the end of the same record. In another approach (which I prefer) BLOB data would have its own HP_DATASPACE with variable-size entries. 4. In a more sophisticated implementation, some space can be saved even with all fixed-size columns if many of them have NULL value, as long as these columns are not used in indexes 5. In variable-size format status should be moved to lower bits of the "next" pointer. Pointer is always aligned to sizeof(uchar*), which is at least 4, leaving 2 lower bits free. This will save 8 bytes per chunk on 64-bit platform. 6. As we do not want to modify FRM format, BLOCK_SIZE option of "CREATE TABLE" is saved as "RAID_CHUNKSIZE" for Heap Engine tables. */ static uchar *hp_allocate_one_chunk(HP_DATASPACE *info); /** Clear a dataspace Frees memory and zeros-out any relevant counters in the dataspace @param info the dataspace to clear */ void hp_clear_dataspace(HP_DATASPACE *info) { if (info->block.levels) { VOID(hp_free_level(&info->block,info->block.levels,info->block.root, (uchar*) 0)); } info->block.levels=0; info->del_chunk_count= info->chunk_count= 0; info->del_link=0; info->total_data_length= 0; } /** Allocate or reallocate a chunkset in the dataspace Attempts to allocate a new chunkset or change the size of an existing chunkset @param info the hosting dataspace @param chunk_count the number of chunks that we expect as the result @param existing_set non-null value asks function to resize existing chunkset, return value would point to this set @return Pointer to the first chunk in the new or updated chunkset, or NULL if unsuccessful */ static uchar *hp_allocate_variable_chunkset(HP_DATASPACE *info, uint chunk_count, uchar* existing_set) { int alloc_count= chunk_count, i; uchar *first_chunk= 0, *curr_chunk= 0, *prev_chunk= 0, *last_existing_chunk= 0; assert(alloc_count); if (existing_set) { first_chunk= existing_set; curr_chunk= existing_set; while (curr_chunk && alloc_count) { prev_chunk= curr_chunk; curr_chunk= *((uchar**)(curr_chunk + info->offset_link)); alloc_count--; } if (!alloc_count) { if (curr_chunk) { /* We came through all chunks and there is more left, let's truncate the list */ *((uchar**)(prev_chunk + info->offset_link)) = NULL; hp_free_chunks(info, curr_chunk); } return first_chunk; } last_existing_chunk = prev_chunk; } /* We can reach this point only if we're allocating new chunkset or more chunks in existing set */ for (i=0; ioffset_link)); *((uchar**)(prev_chunk + info->offset_link)) = NULL; hp_free_chunks(info, curr_chunk); } else if (first_chunk) { /* free any chunks previously allocated */ hp_free_chunks(info, first_chunk); } return NULL; } /* mark as if this chunk is last in the chunkset */ *((uchar**) (curr_chunk + info->offset_link))= 0; if (prev_chunk) { /* tie them into a linked list */ *((uchar**) (prev_chunk + info->offset_link))= curr_chunk; curr_chunk[info->offset_status]= CHUNK_STATUS_LINKED; /* Record linked from active */ } else { curr_chunk[info->offset_status]= CHUNK_STATUS_ACTIVE; /* Record active */ } if (!first_chunk) { first_chunk= curr_chunk; } prev_chunk= curr_chunk; } return first_chunk; } /** Allocate a new chunkset in the dataspace Attempts to allocate a new chunkset @param info the hosting dataspace @param chunk_count the number of chunks that we expect as the result @return Pointer to the first chunk in the new or updated chunkset, or NULL if unsuccessful */ uchar *hp_allocate_chunkset(HP_DATASPACE *info, uint chunk_count) { uchar* result; if (info->is_variable_size) { result = hp_allocate_variable_chunkset(info, chunk_count, NULL); } else { result= hp_allocate_one_chunk(info); if (result) { result[info->offset_status]= CHUNK_STATUS_ACTIVE; } return(result); } return(result); } /** Reallocate an existing chunkset in the dataspace Attempts to change the size of an existing chunkset @param info the hosting dataspace @param chunk_count the number of chunks that we expect as the result @param pos pointer to the existing chunkset @return Error code or zero if successful */ int hp_reallocate_chunkset(HP_DATASPACE *info, uint chunk_count, uchar* pos) { if (!info->is_variable_size) { /* Update should never change chunk_count in fixed-size mode */ my_errno=HA_ERR_WRONG_COMMAND; return my_errno; } /* Reallocate never moves the first chunk */ if (!hp_allocate_variable_chunkset(info, chunk_count, pos)) return(my_errno); return(0); } /** Allocate a single chunk in the dataspace Attempts to allocate a new chunk or reuse one from deleted list @param info the hosting dataspace @return Pointer to the chunk, or NULL if unsuccessful */ static uchar *hp_allocate_one_chunk(HP_DATASPACE *info) { uchar* curr_chunk; size_t length, block_pos; if (info->del_link) { curr_chunk=info->del_link; info->del_link= *((uchar**) curr_chunk); info->del_chunk_count--; return curr_chunk; } block_pos= (info->chunk_count % info->block.records_in_block); if (!block_pos) { if (hp_get_new_block(&info->block,&length)) { /* no space in the current block */ return NULL; } info->total_data_length+= length; } info->chunk_count++; curr_chunk= ((uchar*) info->block.level_info[0].last_blocks + block_pos * info->block.recbuffer); return curr_chunk; } /** Free a list of chunks Reclaims all chunks linked by the pointer, which could be the whole chunkset or a part of an existing chunkset @param info the hosting dataspace @param pos pointer to the head of the chunkset */ void hp_free_chunks(HP_DATASPACE *info, uchar *pos) { uchar* curr_chunk= pos; while (curr_chunk) { info->del_chunk_count++; *((uchar**) curr_chunk)= info->del_link; info->del_link= curr_chunk; curr_chunk[info->offset_status]= CHUNK_STATUS_DELETED; if (!info->is_variable_size) { break; } /* Delete next chunk in this chunkset */ curr_chunk= *((uchar**)(curr_chunk + info->offset_link)); } }