Deferred unique indexes with GRDB

Database+withDeferredUniqueIndexes.swift

import GRDB

extension Database {
    /// Executes the provided closure with deferred unique indexes
    func withDeferredUniqueIndexes(_ execute: () throws -> Void) throws {
        try inSavepoint {
            // Fetch all unique indexes, but primary keys.
            let uniqueConstraints: [(table: String, index: IndexInfo)] = try String
                .fetchAll(self, sql: "SELECT name FROM sqlite_master WHERE type = 'table'")
                .filter { !Database.isGRDBInternalTable($0) }
                .flatMap { table in
                    try indexes(on: table)
                        .filter(\.isUnique)
                        .filter { try $0.columns != primaryKey(table).columns }
                        .map { (table: table, index: $0) }
                }
            
            // Drop indexes
            for constraint in uniqueConstraints {
                try drop(index: constraint.index.name)
            }
            
            // Execute closure
            var executeError: Error?
            do {
                try execute()
            } catch {
                executeError = error
            }
            
            // Restore indexes
            var restoreError: Error?
            do {
                for constraint in uniqueConstraints {
                    try create(
                        index: constraint.index.name,
                        on: constraint.table,
                        columns: constraint.index.columns,
                        options: .unique)
                }
            } catch {
                restoreError = error
            }
            
            // Throw first error
            if let error = executeError ?? restoreError {
                throw error
            }
            
            // Commit savepoint
            return .commit
        }
    }
}