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MDEV-14275 Improving memory utilization for information schema

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SELECT queries on information_schema table will now not store data
in the temporary table for columns that are not used in the query.
This can drastically reduce memory when there are many rows involved
in a query, like using
SELECT table_name FROM information_schema.table
on a system with many tables.
The difference for the above query is about 14K per existing table.

The code works this way:
- Create a bitmap for all information_schema fields used in the query
- For BLOB and VARCHAR fields that are not use, create a Field_null field.
This commit is contained in:
Monty
2018-03-26 17:22:41 +03:00
parent 33fa6abd02
commit e2664ee836
3 changed files with 97 additions and 13 deletions
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12
sql/sql_select.cc
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@ -16685,11 +16685,15 @@ Field *Item::create_field_for_schema(THD *thd, TABLE *table)
{
Field *field;
if (max_length > MAX_FIELD_VARCHARLENGTH)
field= new Field_blob(max_length, maybe_null, &name,
collation.collation);
field= new (thd->mem_root) Field_blob(max_length, maybe_null, &name,
collation.collation);
else if (max_length > 0)
field= new (thd->mem_root) Field_varstring(max_length, maybe_null, &name,
table->s,
collation.collation);
else
field= new Field_varstring(max_length, maybe_null, &name,
table->s, collation.collation);
field= new Field_null((uchar*) 0, 0, Field::NONE, &name,
collation.collation);
if (field)
field->init(table);
return field;