pgsql: Fix interaction of log_line_prefix's query_id and log_statement

Fix interaction of log_line_prefix's query_id and log_statement

log_statement is issued before query_id can be computed, so properly
clear the value, and document the interaction.

Reported-by: Fujii Masao, Michael Paquier

Discussion: /messages/by-id/YHPkU8hFi4no4NSw@paquier.xyz

Author: Julien Rouhaud

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/db01f797dd48f826c62e1b8eea70f11fe7ff3efc

Modified Files
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doc/src/sgml/config.sgml | 10 ++++++++++
src/backend/utils/activity/backend_status.c | 1 +
2 files changed, 11 insertions(+)