Re-used index after ALTER COLUMN TYPE shouldn't change relfilenode

[lss602726449]

During ALTER COLUMN TYPE of an indexed column, we check if the index can be re-used. If so, we preserve the existing relfilenode instead of removing it. However, in GPDB the preservation is done on QD and the relfilenode is packaged in another ALTER TABLE subcommand to be handled later. So QE might make its index to have QD's relfilenode, which could cause data missing/corruption errors.

The fix is for QE to go through the generated index command, and replace QD's relfilenode with its own.

fix #ISSUE_Number

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