What do the values after `execute fetch from` mean in postgresql query logs?
S_26 is a statement name and C_775 a portal name in the extended-query protocol. 

See https://www.postgresql.org/docs/current/static/protocol-overview.html


  In the extended-query protocol, execution of SQL commands is divided
  into multiple steps. The state retained between steps is represented
  by two types of objects: prepared statements and portals. A prepared
  statement represents the result of parsing and semantic analysis of a
  textual query string. A prepared statement is not in itself ready to
  execute, because it might lack specific values for parameters. A
  portal represents a ready-to-execute or already-partially-executed
  statement, with any missing parameter values filled in. (For SELECT
  statements, a portal is equivalent to an open cursor, but we choose to
  use a different term since cursors don't handle non-SELECT
  statements.)


To find the query bound to this statement, look upper in the log for the corresponding bind entry.

If it doesn't exist, you might have to increase the logging level to DEBUG2 through log_min_error_statement, or set log_min_duration_statement to 0.