In Rigveda, Tvashta performed a Yajna to kill Indra after he got to know that his son Visvaroopa was killed by Indra. This yajna ensued birth of demon Vritra.
In Yajna a sacrificer recites some verse from Rigveda to please a deity. Since most of the verses are dedicated to Indra, I assume he pleased Indra to kill Indra Himself.
Also, since the episode has not happened at that time, most of the RigVedic verses that praise Indra's deeds of killing Vritra has not been revealed yet! So Tvashta has less verses to praise Indra.
How is it possible? Does this imply that before Rigveda there were different set of texts used to perform Yajna?