What is Meshichah of Chesed?
Radak: It is leaning, like "va'Yet Elav Chased" (Bereishis 39:21).
Who are "Yod'echa", and where will they receive Hashem's Chesed?
Radak #1: Tilt Chesed to those who know You in this world, so they can engage in Chachmah and Mitzvos without worldly distractions.
Radak #2: This refers to the world to come. "Yod'echa" are Chachamim who engage in Torah, Mitzvos and Divine Chachmah to know the Creator according to their ability.
Malbim: They are those who are not enticed by Pesha's claims. They make their paths straight, both to know Hashem and in deed. Since they know that Hashem conducts miraculously according to the time and deeds, via this He draws His Chesed on them.
Who are "Yishrei Lev"?
Radak: They go with a pure heart and straightness. They engage in Torah and Mitzvos, but they did not reach Divine Chachmah to know the Creator, only via tradition. David requests that Hashem extend to them His Chesed, extra good, to Yod'av, and Tzedakah, the proper amount of good, to Yishrei Lev.
Malbim: Their hearts are straight to fulfill His Mitzvos without veering or doubts. Deeds Bein Adam la'Makom are called Tzedakah in Tanach; so Hashem extends to them His Tzedakah.
