Issue type:

[ x ] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ x ] sqlite
[ x ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[ x ] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

Given the following entity:

@Entity()
export class Test {

    @PrimaryColumn()
    id: number;

    @Column({ nullable: true, precision: 6 })
    startedAt?: Date;

    @Column({ type: 'decimal', precision: 5, scale: 2 })
    value: number;

}

Running a migration on a newly-synced SQLite/sql.js DB results in the following uneccessary migration commands:

[
     {
        query: 'CREATE TABLE "temporary_test" ("id" integer PRIMARY KEY NOT NULL, "startedAt" datetime(6), "value" decimal(5,2) NOT NULL)',
        parameters: undefined
    },
    {
        query: 'INSERT INTO "temporary_test"("id", "startedAt", "value") SELECT "id", "startedAt", "value" FROM "test"',
        parameters: undefined
    },
    {query: 'DROP TABLE "test"', parameters: undefined},
    {
        query: 'ALTER TABLE "temporary_test" RENAME TO "test"',
        parameters: undefined
    }
]

This is caused by a false positive in the findChangedColumns() method:

because the precision and scale arguments are not correctly inferred when reading the table. Then length property, however, is correctly inferred:

All that is missing is to do the same for precision & scale.

I intend to submit a PR shortly.