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docs(router): Explicitly document use of get() and insert() for schema merge 

Co-authored-by: Dom <dom@itsallbroken.com>
pull/24376/head
Fraser Savage 2023-04-26 11:36:15 +01:00
parent 5a9c68e428
commit c837a6e8dc
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1 changed files with 13 additions and 4 deletions
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17
router/src/namespace_cache/memory.rs
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@ -85,10 +85,19 @@ fn merge_schema_additive(
let old_table_count = old_ns.tables.len();
let mut old_column_count = 0;
// Table schema missing from the new schema are added from the old. If
// the table exists in both the new and the old namespace schema then
// any column schema missing from the new table schema are added from
// the old.
// Table schema missing from the new schema are added from the old. If the
// table exists in both the new and the old namespace schema then any column
// schema missing from the new table schema are added from the old.
//
// This code performs get() & insert() operations to populate `new_ns`,
// instead of using the BTreeMap's entry() API. This allows this loop to
// avoid allocating/cloning the table / column name string to give an owned
// string to the entry() call for every table/column, where the vast
// majority will likely be already present in the map, wasting the
// allocation. Instead this block prefers to perform the additional lookup
// for the insert() call, knowing these cases will be far fewer, amortising
// to 0 as the schemas become fully populated, leaving the common path free
// of overhead.
for (old_table_name, old_table) in &old_ns.tables {
match new_ns.tables.get_mut(old_table_name) {
Some(new_table) => {