fix generated columns not being discovered #808

Job	Run time
Unit Test	1m 30s
Clippy	1m 14s
Rustfmt	15s
Compile SQLite (discovery/sqlite)	1m 23s
Compile SQLite (live/sqlite)	1m 13s
Compile MySQL (discovery/mysql)	1m 2s
Compile SQLite (writer/sqlite)	1m 6s
Compile MySQL (writer/mysql)	1m 6s
Compile MySQL (live/mysql)	1m 1s
Compile Postgres (discovery/postgres)	1m 0s
Compile Postgres (writer/postgres)	1m 6s
Compile Postgres (live/postgres)	1m 6s
MariaDB (10.6, live/mysql)	44s
MariaDB (10.6, writer/mysql)	49s
MySQL (8, live/mysql)	57s
MySQL (8, discovery/mysql)	59s
MySQL (5.7, writer/mysql)	55s
MySQL (5.7, live/mysql)	56s
MariaDB (10.6, discovery/mysql)	44s
MySQL (8, writer/mysql)	52s
MySQL (5.7, discovery/mysql)	58s
Postgres (14.4, live/postgres)	1m 3s
Postgres (14.4, writer/postgres)	1m 1s
Postgres (14.4, discovery/postgres)	1m 1s
SQLite (discovery/sqlite)	21s
SQLite (live/sqlite)	22s
SQLite (writer/sqlite)	23s
	25m 7s

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